Taking into account was Ed T & MEngr have shared I would do the following: disconnect the torque converter from the flywheel & test temperature to completely rule out trans, then remove counterweight, open hood & check temperature to rule out air flow restrictions. Seems to me that the H25 engine doesn't seem to have a overheating problem in general- just when installed in this chassis.

I appreciate the feedback from everyone that has posted on this form. Will post more info. later.

I thank both Ed & MEngr for your replies. I appreciate your feedback.

bbforks;
To answer the question. The Optimum series truck has a somewhat marginal cooling system. It is often in need of TLC and "maintenance" to assist in the reduction of overheating issues.
To date the most unusual cause of an overheating engine that I have run across, has been replacement of a +400.00 USD that had restricted cores. To date I have had 62 different causes for overheating in LPG power units of industrial forklifts. If you are new at working with forklifts, some important facts to keep in your thoughts are 40% of heat waste from the engine is removed through the engines walls. The cooling air flow is from the floor, up through the radiator and out the tail weight. The smaller the openning in the tail weight the less air will flow.
In this discussion it's menione to watch for a surge in water flow when the thermostat opens and also to remove the thermostat. The H25 Nissan has a bottom by-pass thermostat, whose purpose is to create a mor even temperature between the engine's bottom and top plus determine the coolant flow direction with the engine temperature. Removal of a thermostat causes hot spots around the number 3 and 4 cylinders which could lead to cylinder wall scuffing and/or scoreing.
The thought I keep having is, Greg dealing with an engine that is operationally sound and is it governed correctly? Also, if he elects to remove the transmission for repair I should think it may be wise to find out if the trans pump and 3 packs need repair of replacement.
Greg, a normal and well tuned H25 hydraulic stall reading will be from 2000 to 2450 RPM. The trans in your forklift is a four element two stage (2 stator) torque convertor. The radiator which you replaced, does it have a part number 21460-86180? If the radiator number is different, I would question. It is an in-line 4 tube with serpintine metal fins designed for excellent cooling capabilities.

BBforks, just because the trans cooling lines do not share space with the engine cooling system does not really disconnect the engine temp and trans temp completely.
One of the problems with the torque convert stator being bad is that the motor is loaded beyond normal when just running, even more when driving, as if it was having to push past almost locked brakes all the time. (why the motor speed never gets beyond 1650 RPM when fully loaded, when it should be 2300 or so, yet gets the full 2700 rpm when not loaded)

How's the trans temp since radiator cooler by-passed? I saw earlier in post that MEngr has experience with this model overheating-what generally is the problem? Did this truck always run hot or did these symptoms come on out of the blue?
What type of enviroment does unit operate in- hot & humid? I know you said that the cooling system parts have been replaced, but were they inspected against the old parts when replaced (radiator core size, fins per inch, # of rows of cooling fins, waterpump vanes in correct direction. I've had waterpumps with the wrong direction fins (presumably for a serpentine belt set up)). Do they have other units of this model at this location?

Lets review:
Greg says he put a external cooler on trans and bypassed the rad trans cooler and it still does it.
The radiator is new.
There has to be something wrong with the coolant flow or air flow.
When the motor is overheated, is the coolant in rad overheated also? If not, then the water isn't flowing properly. I would start it up cold with the rad cap off and watch to see if when the thermostat opens there is a gush flow of coolant happening. If not, then take the thermostat out, temporarily, and see if overheats. If you don't see a fast flow with the thermostat out then something else is obstucting it, or the pump has some sort of problem.
If everything is piping hot, motor and rad, is the air flowing through the rad fins properly?
It seems to me with the trans unhooked, that looking and feeling, or measuring the actual temps, of how the cooling system is working should give you an idea what's wrong.

I agree with the trans being a possibility- but Greg stated he by-passed the trans cooling lines. Any heat from the trans would not be seen by the engine at all with this senario.

I would agree with you Ed. However, I would like to know one additional piece of information to better judge the stall test results, and that would be the engine and electrical systems condition. The way Greg could go now once assured the torque converter's stator sprag clutches failed is to aquire a remanufactured torque converter from Nissan's after market arm, Duralift. The cost wouldn't be all that much, and on the plus side. labor and parts would be covered under warrenty for six months.

However BBforks, it is my experience that slipping clutches (due to an misadjusted inch spool or driver riding the inch pedal) will take at least about 45 minutes (and more likely an hour or 2) of -driving-, to heat up and transfer the heat to the engine cooling system so that it shows up in the engine temp gauge, however the torque converter stator being damaged will show (when using an infrared temp gun type thermometer) serious heat (230f) in the cooling line that comes from the transmission and that heat will be back to [almost] normal as the oil flows back to the transmission, within 15 or 20 minutes of driving (not idle sitting, but moving the truck).
I suggest using the temp gun, shooting the engine a few places (the 'freeze plugs' work best imho), shooting the radiator a few places and shooting the trans at the fittings for the cooling lines.

bbforks;
Clutch slippage heating up the transmission oil.

MEngr- not being a smart a**- just asking- how would a misadjusted or sticking inching spool cause an engine to overheat?

Greg i still think you have a coolant flow problem. I would make sure i had atleast 3 row radiator, correct water pump as the new platinum pump bolt pattern is the same but different shaft and impeller dimensions and thermostat. Be sure to bleed pump housing and use 50/50 mix coolant. Use an infrared laser thermometer and shoot top of rad and bottom of rad. Should be atleast 40 to 50 degree difference in bottom than top. Remember that most of the cooling is acheived on the outer perimeter of rad. Check for dust build up in these areas. The Turkish import rad i think came with two strips of metal running horizontal across core. I had one awhile back in a diesel that eventually got a 10 blade fan. A thermostat is a must as most will overheat without. Check for obstruction in counterweight and muffler is in good shape. These problem can drive you crazy at times.

try getting a laser temperature gun and take various readings around the motor
i just had a thought
could this possibly be a partially clogged head or block?
If you see abnormal hotspots in the head or block that might be an indication.

You could still be getting some circulation in the radiator because of the bypass circuit but if the head is clogged partially it could cause this problem.
I'm not discounting edwardst's suggestion because it is a high probability, in fact take temp readings on the tranny too, i just had this thought while sitting here brainstorming.

***edit***
Ok i got caught up in the rheteric and forgot he already said the trans temp is running at 230deg as well...
So does the engine cause the trans to heat up? or is the tranny heating up first and causing the engine to overheat?
MEngr does have a good point about the inching spool adjustment also, although one would think that this would cause a noticeable difference in drive performance if its heating this badly?

It may ultimately require you to start pulling systems apart and examining them more closely to find this problem.

Just a thought..... thinking outloud here ;o)

Greg;
Were you able to find what the engine's speed is when the mast is tilted back completely and held long enough to read it.
What we see here will verify the cause of your engine's issue. I'm inclined to think you are having a problem with the inching control either binding or misadjusted.