There have been several threads on this, try a search or two for more info-

Of course what engine you want will depend on what you want to do with the truck and how much you want to spend-

You can probably find a used chevy 292 inline six that'll bolt up to the original bellhousing. There are probably lots of performance parts for the 292 out there since it came in lots of vehicles, including camaros. Also, since the engine is pretty common you might find a used one at a junk yard for fairly cheap.

Many people will say that if you are going to hotrod the truck you might as well just go with a small block chevy 350 or 400- They are probably the most common engine in the world, hot rod parts are everywhere for them and consequently they are cheap to build/improve/fix/buy.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi