Bob,
STAY GMC!!!!
The jimmy is a real TRUCK motor!
Anyway, I put a 261 in my '50 GMC 1/2 ton for awhile. It's not hard to do, just fab the front mount but you will have to cut a little out of the cross member to clear the front mount bolts. Personally, I'd stay GMC. Much better performance potential than the chevys (just look at the chamber). I'm puttin' a 302 in mine.
My .02
