I suggest finding a wrecked Explorer. Pull the engine, transmission, computer and wiring. Rebuild the 4.0L. There are actually some decent performance options for the 4.0L. You can get different heads and / or pistons to get higher compression (some combinations too high for pump gas), and a couple Comp cams for it. With headers you'd have a really decent motor, that bolts up.

Of course there's nothing wrong with the 302 option. I've heard that you cant swap a 302 into a 4x4, but that may be wrong. I've also heard that with a 302, things get, um, cramped under the hood.

For the 2.9L, you could hop it up a little bit. World Products makes some heads that do flow better than the stock heads. A little porting and cleanup would help those too. The later non-EGR Rangers had a more agressive cam than the Bronco 2's, so a swap is possible there too. Find a throttle body from a 1986 or 1987 Bronco 2, as it is bigger than the one you have. Put a set of headers on and it should run good.