It is not all that difficult, if you have rebuilt other clutch type transmissions, it is rather a simple transmission, just heavy duty.
BUT if you value your time, and have other work that will make you money, I think I would most likely just consider a reconditioned trans from someone like American industrial Trans (you can search that name on this site, I am pretty sure they have a link).

Hi Edward, yes I checked that the clutch disc is free when the clutch pedal is all the way to the floor. When reading the replies for this problem, it really sounds like the synchronizers are bad. What kind of job would this be? Would I need a manual? Are the parts readily available? Expensive? Thanks Again

As far as the grinding- this trans has no synchros- try putting the trans in neutral with the clutch depressed, engine idling, for about 20 seconds. Then try putting in forward or reverse & see if it grinds. If it doesn't there's no problem, if it does, then the clutch disc is binding in the pressure plate assy & further diagnostic work will be needed.

As far the the lpg hissing, these older units usually use an electric shut off solenoid assy for the fuel which gets its power source directly from the ignition coil. If the lpg system is anything but Impco the fuel will flow right through the regulator & carb & come out the air cleaner assy if the ignition is on but the engine isn't running. Installing a vacuum actuating valve (mounted on the intake manifold-wired in between the ignition coil & lpg solenoid) will solve the hissing noise when the ign. key is on & the engine isn't running.

As far as the milky oil, there are multiple reasons that may happen (crack in head/block, faulty head gasket, etc) but the most likely cause is that the engine isn't run long enough to cook out the condensation in the crankcase. I would change the engine oil (use SAE 30), make sure a 180* thermostat is installed & run engine at least an hour straight on a weekly basis for a month. After a month check to see if the oil is turning milky. If it is, check the level on the dipstick very carefully. If the level seems to be creeping up the dipstick, there is indeed a crack/ faulty head gasket present & further diagnostic work will be needed

If the oil in the bell housing got any water in it, it will get sticky. Maybe try changing that with some ATF.

As far as the milky oil, these units do this if they are only run for 10 minutes or less all the time, as the engine never gets hot enough to 'cook' the normal condensation out of the crankcase. But it can be a head-gasket or cracked block or head, if the unit is one that was running and working for a few hours everyday.
As far as the transmission grinding, a couple of things, are you sure the clutch is disengaging completely? Some of these units [especially after sitting for a very long time] wind up with material from the friction plate stuck on the pressure plates. if my memory serves me correctly, (there is no reason to expect me to remember clearly,,, it was a long time ago when this unit was young and I worked for a Cat forklift dealer) this clutch is one that can be changed without pulling the trans out by removing the scavenger pump on the back end of the trans input shaft and removing the shaft that drives the pump. you should be able to check the play of the clutch plates through the top cover while operating the pedal and linkage. BUT if the clutch friction plate is freely clearing the pressure plates when the pedal is down to the floor, then {as Swoop says} it will almost for sure be the synchros in the trans.

Hi fred,
well the grinding when you put it in gear is most likely the syncronisers being worn. Putting in a heavier oil may help quiet it some but instead of using the 80w90 try the ligher 70w oil. If the oil is too thick it might make it harder to shift when cold.

As for the propane problem it sounds like the converter or maybe the fuel lockoff, my guess is it has a century converter? the lockoff may be built into it.
I checked my cat service and parts program but that truck is so old it doesnt show any service information, only parts.
sorry i cant help more, maybe someone else here may have an old hardcopy of a service manual laying around.