I think you'll be very happy with the 700R4 if mileage is your goal. The drive ratio in 4th gear (overdrive) is .7 to 1 versus 1 to 1 with a TH350. In other words, for every .7 revs of the engine, you get one full rev of the output shaft on the tranny...nice!
An additional note. Although some people don't like the 700 for performance, they can be built for some serious applications. There are a number of different accessories and parts you can buy to optimize your tranny for your particular application. For instance, they have a wiring kit for the lock up to reduce slippage. They also have what looks like a little cylinder that goes inside the tranny and allows you to stay in 4th gear with the hammer dropped (if you want to). Otherwise, the stock ones will drop back to 3rd gear for acceleration...I think the 'vette ones might not do this.
Anyway, just "google" some of these parts and I'm sure you'll find something that will help.