if you're looking more for economy than power, I'd look for a 7.25 or 8.25 axle. less mass, which is a good thing. also, I'd probably look at 2.94-3.23 gears with the OD. I'd also look at an A833OD instead of the A500 (less mass, and planetary geartrains are not as efficient as regular bevel geartrains)

another option for highway efficiency would be a slant 6 A833 (3.09 first gear, 1:1 4th) with like a 5th ave/diplomat 2.2-2.45 rear gear. technically, the least amount of geartrain loss in a manual tranny is in direct drive), and the 3.09 first will make take off tolerable.

personally, I'd go a 318 roller as a base, either using the 302's that came on it, or the mag heads. I'd use a hughes HER9204AL (192/204@.050, 114LSA, .459/.471" lift with 1.5's and .490/.502" lift with 1.6's--that's some pretty fast ramps)for the cam, and KB167's (or equivalent KB forged) for pistons. frankly, I don't think rod weight would come into play that much, I'd probably use the magnum stockers. that should be right about 10:1, I'd go super tight on quench, probably ~.035"....I bet it would still run on pump premium, you might have to play with where you install the cam, or timing. thermal barrier coat the tops of the pistons and the combustion chambers, and pump gas should be fine.

couple that with a standard eddie performer intake (as they seem to make the most torque down low of the small block 4bb intakes, at the expense of upper RPM power), headers and a small t-quad. or if you use the mag heads, I'd use a stock mag intake, smooth out the throttle body, and run the whole thing with a megasquirt, or GM TPI computer....you can get way more creative on fueling and timing curves with EFI to max out economy.