The jimmy has a better combustion chamber design, and the valves go straight up and down instead of being cocked like in the 235/261. The good thing about the 261 is that intakes are cheaper and the block is shorter.
If clearance isn't a problem, I would go for the GMC.
I'm replacing my 261 with a GMC but its a 302. There's no contest there. The GMC engines cost more to rebuild than the 261, at least in the US.
