Ok, here's the deal.
a) The diesel swap will certainly have a high *WOW* factor but I'm getting the vibe that you're a cost conscious sort of guy and for the coinage you'll lay out to do this you'll just never recoup it in fuel savings. And while sure the diesel will give you respectable torque, I REALLY don't think you're going to get a 4-cyl deisel that will outgun your current 351 as far as torque goes., just not gonna happen without swapping in a turbo-diesel and whats *that* going to do to your budget ? Swaps like this are for guys with a whole lot of disposable income.
B ) Sticking with your current mill & swapping in an AOD is definately the cheapest route to increased milage. The AOD is a mechanical overdrive tranny so you won't need a computer to run it. If you go this route you have basically two choices, either find a donor tranny and install it or buy a rebuilt unit froma vendor like TCI and install that. Something froma vendor will already be rebuilt & upgraded so there shouldn't be any worries there, if you grab one from a donor make sure it's from an 88' or newer because those boxes incorperate all the upgrades Ford made in the valve bodies & such over the years, earlier trannys are weaker & more prone to failure. Now you can ALSO swap in something later like an AODE/E4OD but those are electronic OD trannys and you'll need an aftermarket stand-alone computer to make it work without the correct factory engine & computer.
c) the 300/AOD swap. The far & away CHEAPEST route to go with this one will be to snag a donor truck. You can get nickle & dimed to death buying all the little bull$hit parts like frame perches and accessory brackets. Just find a parts truck and buy it, you'll have everything you need and can probably part out what you don't to help recoup some of the investment. This option also gives you the option of using a later model electronic transmission since you'll also be swapping the fuelie engine, harness and computer so there's no reason to shy away from a late model electronic lock-up overdrive transmission that will give you the best possible milage and gearset. As far as chosing years, well that one depends. If you go the parts truck route you can grab just about anything and I'd take advantage of that fact to get a reasonably late model low milage machine with all the nice goodies. The real bitch with the electronic lock-up transmissions is that Ford piggybacked the transmission processor onto the engine computer so you really have to have the whole package for the tranny to work right unless you spring for an aftermarket stand-alone. Most places around the country you can find a nice parts truck for less than $2K or $3K and you can figure on getting maybe half of that back if it's a 4x4 and you part out what you don't use.
And just for $hits & giggles let me throw THIS idea out there. You're obviously a guy who appreciates the WOW factor in both his truck (which, if we can be honest here probably had a lot to do with your choice of an EB in the first place <grin>
AND with an engine swap. OK, so an I6/300 is a pretty mundane engine, you have the room under your hood to fit a newer modular motor. The 97' & newer F150's came with them and swapping in a modular would certainly give you a huge WOW factor on Saturday nights and out on the trail, on TOP of the fact that you'd be getting a *very* fuel effecient engine/tranny combo. The 97' F150 2-valve 4.6L V8's were rated at 248HP/284 torque, the 3-valve Explorer engines were 300Hp/320 torque and the 4-valves ran 302/318. Those numbers are for the 4.6, the 5.4 Lightning engines ran 380/450 but that's getting a little over the top. My point was that you could easily find a modular engined Explorer and swap the engine/tranny for a dramatic increase in milage and a KILLER engine compartment. Just something to kick around.
Dunno if that answered all your questions but it should give you a good start, lemme know if I missed anything you were particularly interested in.
