in those years, i'm partial to the standard-issue 258, but add HEI and a 32/36. For you, though, since there is no motor in it now, i'd go with a cheby 350 swap. Lots of documentation, easy and cheap to build up, and plenty of kits available for motormounts, etc. Plus you can probably find a '5 with a 350 swap done in it near you; it really helps sometimes to "see" how things go together, especially since you have no motor at all installed now.
Personally, my next Jeep is going to get a 4BT cummins (4 cyl Diesel).