So I determined that my big old 1979 Delta 88 with 205/75R15 tires (27.1 inch diameter), 2.41 rear axle, 1.00 ratio in top gear, and 1475-1775rpm cruising speed (determined from a 4 inch stroke), gives me 49-59mph as my ideal speed to drive for efficiency.
The speed limits around town are 35, 40, 45, and 50. 60 in between towns, 65 on the interstate and east of Alvarado on US 67. So driving 60 as I have been doing is about as efficient as it needs to be. I'll be driving 50mph as much as I can from now on. Should I choose to change out my transmission for a 700R4 overdrive later on, my ideal cruising speed would rise to 70-85mph. I guess there's no point in that since I can't drive those speeds around here. Car might as well stay stock and I'll keep it tuned. I'm having no trouble getting 20-22mpg in 50-60mph cruising mode, and for a 4000lb car that seats six and never leaves the house unless it's loaded full up with kids, I think it'll do.
I checked out some various different cars. My wife's car does best from 56-67mph and the 1999 Suburban we're looking at is ideal from 62-73mph. Sounding good to me.