hmmm..
where do you live? (area)
i hear of people cracking 19 mpg with the inline 300 and 302/351 on occasion...but rarely have seen it. but i know it can happen as i have proved out a oem bronco with 351 before at 21 mpg solid hiway cruise..
17 plus mpg from a 302 in a 4x4 that is stockish in ride height and tires is easily attainable.
problem is, finding a low mile 300/6 or windsor engine.
351 and 300 been dead and gone since 97
.....302's since 2001.
using running donors lowers cost. they are out there.....like stated...CAREFUL SHOPPING.
this is why i would be tempted to move to a gm powertrain or the current ford v6 150 setup....easily package-able and easy to get sub 60k miles. the ford electronics are a trainwreck though...
careful tuning of the explorer v8 setup if your willing to get a longblock would work really well. the 4r70w trans is really a stout unit and the awd t case may be acceptable in your application overall. this can literally be plug and play with a 99.
a benz diesel and awd drivetrain would be cool though....when the v10 vw phaeton diesel came to life years ago it had my undivided attention....i pursued it but it was way too far out of my pocket.
given the choice between a stock tune explorer 302 and a 6.2 diesel with turbo i would take the diesel. but i have a larger then average 4x4 ranger.
many paths....but 17 mpg in a 2wd ranger that was nutless would piss me off enough to do something too.
there are many options so think about it...the 4d55 is a diesel that came in early model rangers...sokken in california deals with the jdm market which may offer some options to consider...
the explorer offers the bolt in solution with the sla 4x4 chassis and your 99 body so it will be hard to pass up.