all of the above have been done just FYI
the 3.8L is RARE but there was a Ranger or two running around with the 3.8L Souper Coupe SC version, the problem with the 3.8L conversion is why not just go with the cheaper to build V8 5.0L? same bolt pattern just two more cylinders
I believe the prerunner Ranger that did have a 3.8L SC eventually went 5.0L after battling the SC and custom setup for a bit, of course his engine was also pushed back into the cab for desert driving...
4.0L with C4 is a great combo assuming the C4/5 is built right
You will lose overdrive
Heck these days they are even doing the SOHC 4.0L with a C4/5 behind it in the So Cal 2wd scene
The AOD behind a 4.0L is questionable, the adapters you can get now are nice but IMO why not just go V8? Power loss would be a drawback. But thats just MO, with the right 5 speed and t case this could be an excellent setup
the v8 has been done from Carbed to turbo, heck even Ford put a 5.4L lightning drivetrain into a Ranger, anything is possible when it comes to V8 conversions.
When planning a conversion for a 4x4 I usually like to say pick a transfer case, then work forward from there. The t case you pick will likely drive your transmission choice, which can then drive which v8 choice to go with based on what (if any) computer is needed to control the auto transmission.
the Explorer GT-40p 5.0L with 4r70w is a VERY desirable setup IMO if you can swing it. You get a late model pushrod 5.0L with GT-40 heads and intakes, (roller lifters, the short front serpentine dress, DIS, MAS, and OBD-II) a computer controlled wide ratio auto trans that can take the abuse and can be adapted to many different t cases including Atlas, Dana 20, BW1350-54, BW 4406, and more.
Just my 02