I'd use one of the may on line calculators and see the RPM numbers you are going to have. And remember changing tire diameter does the exact same thing as changing rear gears. So, both those need to be factored. Anyway, once you get an RPM number for highway speeds (65 MPH) use whatever gear in your present setup that comes closest to that RPM at that speed. I'll guess about 3rd gear (manual) 2nd (auto). Drive it for 10 miles in that 4,000 or so RPM range and ask yourself if you REALLY want that all the time.
Years ago I was given a Datsun 510 that has 3.90 gears. Coupled with 215-50-13" tires (remember, as I said they affect ratio too) the car was 4,000+ RPM on the highway. In normal driving I was shifting into 3rd gear before I even crossed an intersection. There was no presence of acceleration, just an all too quickly winding out engine. Thankfully early Z-cars had 3.36 gears and they made the car so much more drivable.
In my Turbo Pinto application my 8" rear had 3.00 gear. Coupled with a T-5 and 175-70-13" tires the car seemed to never have the right RPM for any posted speed regardless of the gear selected. Going to 3.40 helped significantly. The car is now drivable.
Tires size aside for the street 3.25-3.55 seems to be an appropriate range. I'd stick with the 3.40 and see what it is like. With the 6-3/4" rear you not only have the cost of the gears, but properly setting them up as well.