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5.8/c6 or AOD would be easier and more suited for the bronco.
the swap you speak of was done in an older 78/79bronco and took the guy a year or so and some coin. i think it cost him around $5k for the swap and that was doing most the work himself.
Lot of fabrication work to lose nearly half a litre in displacement AND have to swap transmissions and engine/transmission management electrical systems too.
For mileage, go for the AOD. Overdrive will always help in that department. The C6 has no overdrive so highway speeds are going to require higher RPMs. Higher RPMs = more fuel burned.
The AOD trannys closer to the late years of the model will be the better ones (reliability wise), if you decide to go junk yark hunting. Or you could have yours rebuilt by a reputable shop.
know that you must get a flexplate for an 88 ford crownvictoria running the 5.8/aod combo. the 5.0 flexplate will not work. its best to buy this new because most wrecking yards cant honestly tell the difference. If the wrong one is used you will ruin the tranny and have major engine vibrations.