What is the ultimate goal of your build?
1) F-134 would be stock resto
2) A late model 2.5EFI could have a stockish appearance if done right, but with twice the HP and the convenience of fuel injection
3) Buick V6 was a factory option from '67 to '71 and still a popular swap option. Although the original were 225 odd fire, 231 even fire are widely available and have a similar external appearance.
4) Buick V8, especially 350 Buick, will fit if you're looking for something bigger. The benefit of this engine is the forward dist location. Late CJ grille and 304 radiator make installation easy, or there are aftermarket options.
5) Ford 302 or SBC will fit, but more modifications and aftermarket parts are required than with the Buick.
6) Straight 6 will not fit without cutting a hole in the firewall and sliding the engine back, or putting a postal grille on the front. There are too many better swap options available to seriously consider this route.
7) VW Rabbit Turbo Diesel is an awesome swap option that fits like a glove, but rebuildable cores are like Hen's teeth, and runners fetch a mint.
8) Ford 4cyl "Pinto" engines were popular back in the day, but I think the late TJ 2.5 mentioned above would be even better for the EFI.
9) Christian Hazel of JP Magazine put a late GM V6 (3400?) crate engine in his flat fender. Although the late model engines are designed for FWD cars, you can use an older 60deg Camaro/Firebird/S10 bellhousing as an adapter.
10) There is a board member Scott Milliner at www.earlycj5.com
that swapped in a GM-based OMC boat engine complete. Nice looking swap and very practical for an offroad environment.