Not sure how a highly modified 4.0L would get "a lot better gas mileage" than a 302. Hell, not even a little better.
I used to get 16.5 - 17.0 MPG with my 4.0L 5-speed Auto Ranger. When I swapped in the slightly modified 302 4-speed Auto engine, the mileage dropped to an astounding 16.5 MPG. (My truck weight 4500lbs even)What was better, is that was always the MPG, highway, city, etc.. Towing mileage is much better with the V8 than the V6 as well. Now, with the Kenne Bell, I am getting 15.0MPG, but that is due to my accel pedal being stuck to the floor all the time! Good MPG does not always come with the smaller engine. Heck, the 5.4L V8 Expedition nearly always out runs a 4.6L V8.
As was said above, you are biting off more than you can chew. Any serious mod or engine swap should be left to those mechanically inclined. You are asking for alot of trouble if you dive in for any of the above without the knowledge of how things work and how you can improvise. This is especially true for a turbo system. You don't just get a mustang kit and slap it onto an explorer. You are looking at many long nights of trial and error fitting of tubes, sizing turbos and exhaust piping, then the inlet plumbing, not to mention the myriad of supplemental modifications.
Buy a sports car or get a V8 explorer, do aftermarket exhaust and be done with it.
Just my option...