My "new" '74 Duster- or why I need a project like a hole in the head

I have the T-56 magnum for my 69 Dart project. It’s pretty far away from install but deciding rear gears for my 8.75. I’ll probably have a around a 26” tire. I do mostly highway driving and will want this car to be my daily driver. I was thinking 4:10s or 3:91s. The engine will be the blueprint 408 base with cam lift .238 intake .474 exhaust @50-110 degree lobe separation. What gears would you run? Thank you