From what I read, it would like you have a timing problem.
Get the truck warmed up and running and set the timing BY EAR.
Rev the engine up some, maybe 3000 rpm and move the dist more and less advance. The engine should speed up and slow down some.
Set it to where the engine is running the fastest, then back just a little bit.
tighten everything up and drive the truck and see if it helps. If it does, you have a problem with your timing!