Don't know alot about the 258. I understand teh 242 is a 3 main bearing motor - yes only 3!!!!!!!!! This makes for a whippy crank and more HP may not be adviseable.

The early ones made had a lower peak torque number (1800 RPM) versus the later ones (chrysler engineers got in there, moved peak torque up toward 3,000 rom if i recall)

The head on that mnotor probably can't be made to breathe as it is not a cross flow hear (cross flow - intake on one side, exhaust on the other). There is just no room for bigger ports, and they have to be small to start with to even fit.

Go see Jeep sites and see what those looneys are up to. I know a V8 is probably heavier than the 6, but it may be shorter in height and more rearwardly mounted to help weight distribution.

what is your ultimate goal? 1.4 mile, rock crawling, or F150 lightning beater?