Time to let it ride. Did a recheck and all seems fine. Thanks for the help!

UPDATE:

I let the lift run today. Runs like a top. No running issues.

The trans issue. I drained the transmission fluid. I refilled and installed new filter. I blocked it up. Let lift run in fwd and reverse for 15 minutes. I drained it again. Cleaned the filter. Refilled. Repeated the fwd/reverse procedure and drained it again. I refilled. Each time the amount of material was significantly less, and the last time i checked there was no material in the filter. I returned lift to service. I will recheck tomorrow.

I will recheck this am

There is nothing that could have done during install due to The engine only going so far to bolt up to the transmission

there IS "clutch friction plate material" from the transmission that MAY be what you are seeing.

Well that is very good info. Being I just installed the motor, I was a little worries that there was something that I could have done to cause it? At best what I can describe it as is black maybe plastic/graphite pieces. They were running it hard right before it stopped. I have never done a k21 install so the help is golden. I have never been in the transmission on the Nissan family so that is why I am asking about the possibility of the "plastic". I will pick up a couple filters, a valve body gasket and a couple gallons of oil.

my guess about the plastic in the trans filter is going to be; it got sucked into the trans from around the U-joint, you should check to make sure the operators were not using the 'vacuum cleaner' functions, by driving over plastic shrink wrap rather than picking it up. there is no 'plastic' that is native to the inside of this transmission. you are going to have to get this truck up high enough to get underneath and check the U-joint. make sure to check all the bolts are torqued properly on the U-joint, this U-joint turns at a pretty fast rate, and does sometimes let go if even 1 bolt is not properly torqued, I always add red locktite to the bolt threads if they are ever the least bit loose. The U-joint often takes the transmission and/or differential housings with them when they fly apart, meaning the truck needs a trans and diff replacement with no good core for rebuild (high dollar). but it is about the only place plastic can get up inside this trans.

I will check.

Will that cause the plastic shavings in the transmission?

sounds like not getting enough fuel, do you have a black washer inside the truck half of the LPG tank quick disconnect?
you say the truck ran for 4 days? did it run -correctly- for 4 days? if so, I would not blame the install. I would expect something after the install.
you say it "bogs down about every 2 minutes" how long does it "bog down"? does it come right back or stay "bogged down"? does is do it just sitting, or only when driving, or when lifting but not moving forward or reverse?
Have you confirmed the transmission speed sensor is connected? is there any possibility that the VCM or ECM was swapped with another truck while it sat? (installing a pnuematic's vcm into a cushion truck, and/or installing a 3k vcm into a 5k truck makes symptoms somewhat like that, since it has a different number of counts in the trans sensor for different engine speeds, even if the truck is not set to have speed control)

MAPL02A25LV
9u0667

Well I did the install. Did the relearn. Runs good. They had it in service for 4 days. Lift stopped moving. Checked, found transmission filter stopped up with "plastic shaving". Not metal. Checked. Where do I go from here? A little frustrated but really not sure what could have gone wrong on engine install. The lift sat for 7 weeks while engine was out for the remanufacture process.

Not having a complete model and serial number kind of hampers the quality of opinions you will get with your posting.
As I answered in the [hijacked] Raymond thread, there are 2 learn procedures, one of which requires a laptop connection.
I am not sure how many 'ground' connections there are.
Does this slow down happen when sitting still or only when driving? there is a 'known symptom' that happens when the wrong vcm gets installed (one for a 25k capacity will not work in an 15k, and a pneumatic will not work in a cushion truck), where the speed sensor does not match the the math for the number of pulses it ought to have at a given RPM, that is very much like what you describe, but I am pretty sure it only happens while driving, but might be happening just if the wire on the sensor gets knocked off..