MPG Motor Magnum vs. Hemi

If you are really gonna rebuild that 5.2:
Might I suggest aluminum heads,very high compression,a very modest cam to go with that overdrive trans, and fresh-air intake?
I have run 87E10 on 185psi cylinder pressure, and point-to-point, got mileage you might not believe;with a carb, in my 3850pound(loaded), manual-trans,68 Barracuda, and a 1.97 final drive..... with an LA367,even.
No EFI required, when the engine is idling down the hiway at 75mph=1830rpm, mostly on the transfer slots. No; don't try this with a Holley, it's a little tricky. But any spreadbore metering-rod carb will drop you into the zone.
I expect your 3.21x.69=2.21FD and "somewhat heavier" combo won't go 32mpg, but I can see mid 20s easy enough, point-to-point.
But I can also see a rear swap to 2.94s and an A500 (with it's 2.74 low-gear),yielding a similar starter gear, (8.06 vs 7.86) and the new FD (now 2.03) and the smaller displacement 5.2 ....... sneaking up closer to what I was getting. Your tank can't be that much heavier than my barge, can it?
185 psi is a little harder to build in a 318/5.2, than in a 5.9, but with a small enough cam, should be doable. Hyup, should be. In a spread-bore cruiser, I wouldn't be afraid to tickle 195psi with aluminum heads.

Ok some clarification;
IMO
You will get better fuel mileage at 185psi/aluminum heads and a reasonable good tune, including a dial-back timing device,
than you could even dream of getting with 160psi/iron and EFI, perhaps even with computer spark control.
And with no other changes, you will also get more power;the compression increase by itself is worth nearly one cam size.
185psi might sound like a lot, but considering the VE at 2000rpm,with a small cam, it is no strain on the engine or octane whatsoever. And the passing/hill-climbing power will be a welcome relief I know. And in fact your sub-3000rpm performance will approach that of the old stock 413. Whereas your 5.2 at 160psi will not touch it.
Personally, I would leave the 3.21s in there, but I would go with the A518's lower gear ratios. Your 75mph would still be under 2000 with 28s, and the passing power at 60,in second, will be enough to scare you.
Just saying, and trying to get you to think outside the box.