/6 with 4 Speed OD, rear gear recommendations?

I have /6 Dart Lite.
It came from factory with 3 speed and 2.94 in 7.25 rear axle.

I have 8.75 that I want to swap over to. I also have 4 speed OD that is also going in at some point.

Question:
What rear gear would you recommend with the OD?
I drive this consistantly, 2-5 times a week, and most of my driving is highway (25MPH to 65MPH). There is some around town, but it is not huge.

I have a 3.23 and 3.55 at my disposal. Would 2.94 or 2.76 be too far fetched?

I do NOT need monster acceleration in this vehicle. It is staying /6 eventually getting a Grand National turbo with Offy 4 bbl intake, and 4 bbl carb. I like to play to see what MPG I can get.

Opinions?