"...so how hard would it be to swap a Straight 6 into a Bronco anyways?"
Prob. not hard at all, depend. on your abilities.
You would need the I-6 300 Ford eng. (great eng.)
Issues would be the weld to the frame motor mounts to fit the 6.
Then maybe some trannie/transf. alignment with the crossmember for the transf. mount. And maybe some dr. shaft changes.
--- Might be well served to check with Advanced Adaptors as they have adaptors for just about any eng./trans. combo. ---
Having said that., at the end of the day.., I don't know if the mileage increase would be enough to warrant the work & expense. It could take some ten yrs. (of better mileage) to recover the difference.
Look here---
http://forums.4wheeloffroad.com/70/620272/engine/300-ford-inline-6-power-mods/index.html
The 400 is a good mill. freshening it up would likely increase mileage.
I would assume it's poss. to de-tune it a bit and/or sleeve it down to a 360 or so..... you might research that online.
Any off road/trail travel is going to drop mileage..., but then you're usually not going 2-3 hundred miles a day...! More likely 10-50 miles.
It's an "up to the individual" kinda thing.... >

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