First off the only indicator for any problem that occurs on this truck is the MIL light on the dash, if any problem comes up in the system this MIL light will light up and stay on as long as the problem exists. This is a Carb Tier 1 system on this truck so there is no ECM so to speak. It is refered to in the manual as an ECS (Engine Control System). This is a 4G63/4G64 engine setup. Being a GC25K most likely is 4G64, you can verify this by looking on the right side of the engine on the block, there is a flat (machined)place that has the engine model stamped on it.
There are 2 tools available to use to adjust this system, a GSM handset and also the SG15 laptop program. The manual references the laptop program only, i have no setup information relating to the handset interface.
The accelerator pedal can be set using this program, if you want to try and do it by hand good luck, you may get lucky and get it set.
IF there is a timing spocket mounted on the front pulley check the wiring and make sure the harness is not damaged and all connections are good. Generally if this component does not work or the harness is damaged the engine will either not run at all. Might be worth checking though.

Anywho, if you just replaced the motor i would check the mixer and make sure its clean, the diaphram is not damaged. And one other thing is there is a setting that has to be set using the adjustment screw on the mixer, it is hidden behind a plug and is used to set the dwell on the fuel system. If this is not correct the engine will not run correctly. There is a high possiblilty that it was setup to run with the old engine and has been adjusted over time and will need to be reset back to factory setting since this is a new engine.

Now some of the stuff MEngr said about checking vacum lines, make sure they are connected correctly. Also if it has a new throttle box it will have to be relearned to the pedal position sensor. If it is the old one then nothing needs to be done as long as no-one disturbed the pedal sensor setting.

As for the spark plugs? use the ones cat suggests , the current part number for these plugs is 1038083 for the LP engine.
I would concentrate on the pedal sensor setting and the mixer dwell setting, you might find this is where your problem is.

just some things to ponder on while scratching your head...