Dunno how hard core you want to get about MPG, but my experience is that running at LOW rpms gives better mpg even if below the "efficiency peak rpm". But instead of getting lower gears and bigger tires, you can just get SMALLER tires. Again this would be a hard-core mpg play, not a "bad-ass looking truck" play, but smaller tires would also lower it and reduce aerodynamic drag.
Speaking of aero drag, depending on how much driving you'll do at highway speed, that is what will yield your biggest results (assuming the new engine runs well). And by "running well", I mean stock re-build - nothing fancy or exotic. You mentioned a bigger air dam. YES. take it down to the same height above the ground as the lowest hanging stuff under the car. Grille block: again YES. I got 2 mpg just from that. Block off as much as you can & not overheat. You might be surprised and be able to block almost all of it! Looking at your side mirrors.... again, for hard-core mpg and "screw the looks", how about getting some really small car mirrors on there? Then finally, build some sort of lightweight Kamm-back or boattail.
I know you are all into your engine mod ideas, but my experience is that as long as the engine doesn't run like crap and waste lots of gas because of that, the bigger gains will be from aero and hypermiling techniques. Of course, do the lightweight synthetic oil and remove all unneeded parasites running off the fan belt (and the fan itself, probably - replace with an electric one). And same thing with lightweight synthetic gear lube in the differentials, too. - and keep the tires aired-up to max sidewall or above.
Simple things like that will, I'm guessing, add up to more mpg improvement than the most exotic specially engineered high-efficiency engine would.
I'm reminded of MetroMPG's story (I think) about a 1st generation Honda Insight hybrid he got. Thing is - the battery pack was shot & he didn't want to buy a new one, so just disconnected all the hybrid-related stuff and got like 100 mpg out of it as a conventional gas car!!! See, it wasn't the exotic engine (hybrid system) that made the 1st generation Insight so great (although it helped - some), but instead, it was overall shape, size, and design of the car itself. I'll bet you could transplant just about any similar sized, modern efficiency gas engine into a 1st generation Insight and still get stunning mpg with it just because of the CAR, not the engine. So I say if mpg is your main goal, don't put so much effort into the engine, but instead, somewhere it'll REALLY help - like a boattail, air dam, grille block, etc, etc...
