The 235/261 engines are a couple of inches shorter than the GMC, so you'll have to do a little monkey motion on the front motor mount position. Other than that, and possibly a little tweaking on the exhaust pipe and throttle linkage, the Chevy engines will be the easiest to adapt. 250 and 292 sixes WILL NOT be an easy swap. One simple way to free up a badly locked engine is to use a big hole saw to cut the piston tops away just inside the rings, and break the centers out with a BFH and a chunk of round steel bar on the piston tops. They're going to be trash anyway, so there's no point in going to a lot of effort to save them. Nissan Diesel forklift pistons and sleeves can be adapted to a GMC engine instead of paying outrageous prices for custom made pistons.
Jerry
