Rearend oil weights and MPG

never have I used 140 back there; only 75/90 dyno. In my A833 I use 50/50 ATF and 75/90. If you have a slanty then you can use 100% ATF. I only put the EP oil in there to keep my cluster-pin happy with 430 hp.
With an auto, well you have only one choice; but you can put a shift kit in there to reduce your shift-overlap, cuz in your case I am imagining a lotta lotta shifting going on.

10 to 15 mile trips around town means there is no way to make any kind of fuel economy. Especially with a low-compression engine. All you can do is optimize the timing and hope for the best. Around town, I can almost guarantee you will get better mileage with more gear, more than the 2.73s that are often in these old A-bodies. Lord help you if you have 2.45s; those are strictly for hiway cars and 318s!
I would try 3.23s at least. This will make it easier for the engine to get up into the efficiency zone, using less throttle. And the gears following, keep it there. Not until you hit hiway speed will the economy start dropping. And there is an easy cure for that, just slow down. Also, you can optimize the timing for cruise by yanking out the Vcan, making sure it actually still works, and then mod it to bring in a lot more than the feeble 9 or 11 degrees they are often born with; 22 degrees is possible. I think TrailBeast got 24 out of his.
With slantys don't mod the can straight to 24 with numerically low gears; there is a good chance the slanty with 10* initial will detonate at Part Throttle if it sees too much Vcan timing. Take it slow