[84LX89GT]

What i would recommend is from a 5.0L explorer, getting the GT40P heads/intake manifolds (upper/lower), throttle body (if it can be adapted to a truck), fuel injectors with a new mass air meter calibrated for the 24lb injectors, small primary diameter headers with a smooth Y-pipe transition and single 3 inch pipe (maybe 2 1/2"), get a torquey cam for the engine, or maybe even the stock 5.0L explorer one since that is meant for lots of torque. Definately get a new torque converter if this is being used for towing as well as a better trans cooler for the higher stall of the converter.

As mentioned a little port matching/porting goes a long way especially for torque and just overall power output. You may also want to get a new MSD ignition coil, cap rotor and plugs since you're replacing the engine anyway.
It also wouldn't hurt to buy a windage tray, since they're relatively inexpensive and are good for a little bit more power.
Some 1.6:1 roller rockers provide less valvetrain friction for more power as well.

The GT40P heads would probably be perfect for a torquey 302 because of the quick burn chambers and excellent mid range torque band the heads offer with the improved flow. I think the biggest difference made would probably be with the cylinder heads. Again, as mentioned, if you can get some AFR or other aftermarket design you really can't go wrong, you'll get a ton more torque and usable power everywhere, the stock heads just suck.

Basically just improving the burn and reducing pumping losses from the engine (exhaust/heads) should make a noticable difference anything else will be butter.

This is my opinion, hopefully i helped in some way