The GMC 270 might be down on ultimate power, but having been around a few of these engines for a while I'd elect to have the 270 over any other option. ( I had the 270 in my DUKW, my CCKW, and my Chevrolet 1.5 ton <yes it was the 270 not the Chev engine>)
True, it's only a 270 six cylinder, but you can't kill them with a stick, and a big stick at that. Don't forget how many of those engine options you mention are overhead valve, as opposed to those blooming side valve things. Full pressure oiling (unlike the Chev) and six cylinders are easier to wire and maintain than twelve, and if you need bits there were plenty around.
I love my side valve Dodge / Chrysler sixes dearly, but not even the longer and larger Chrysler sixes would tempt me away from the 270 as an engine option.