i had a 3.4 with the AOD, and didnt like it. Didnt have as much pep as it i wanted. Since then i've install 4.11s. I like the low gears, but they do make slightly more noise.
By the way, I run at 2350 on the freeway at 70mph, and got 20mpg with an edelbrock 600. Havent rejetted since installing the holley 650
My guess would be 3.50/3.55:1 or 3.73:1 depending on what's avail. for your diff., you didn't mention if it's an 8" or 9". Your tire height will be a key in this equation. Here's a handy calculator to get an idea of your RPM's at any speed. I made a spread sheet with my tire ht., AOD overdrive ratio (0.67) and speed to see what my RPM's would change to mainly at 70-80mph and ended up going with 3.50:1 for fun and hwy. Use the calc. to figure out your tire ht. first then plug in the numbers, kinda fun to play with, I checked to see what I'd be turning in 3rd at the drags at 100mph, 110mph, etc. out of curiosity.
Don't forget the taller you go (4.11:1) the shorter your 1st gear on up gets also so it'll shift out of 1st sooner during normal driving with too tall gears.
I had my C4 with 3.55 equalock and 225-45-17" @ rear and was cruising @65mph doing 3,000. The AOD swap lowered to 2,600 and still have an exellent pull in 1/4 mile and a quiet ride.