Looking at the pics that are linked by the OP, it looks like his truck might have a Chevrolet engine in it, rather than the GMC. I can't tell for sure which engine it is, though. It does have the 3 vent rocker cover.
Might be a good idea to confirm what engine is in it first. Can you possibly get some better pics of it? also, serial numbers and casting numbers will be usefull.
