Optimal Tire Diameter

[BC10]

Hi Everyone,
What diameter tire will yield optimal performance on my truck???
After searching and reading through many of the topics based on rear differential gear ratio and tire sizes I am still stumped, this may be a result of me being a rookie in regards to rebuilding vehicles. I also tried to use the online calculators but am still stumped.

I have a 1968 short bed c10. According to the previous owner, the engine and tranny (TH400) was transplanted from a 1969 Camaro z28. The original rear differential was a 3.08, which I have since replaced with a 3.42 (much more responsive now).

I am doing a little of city and highway driving, and some light hauling. I recently purchased 15x8 stock rally wheels that will replace the current aftermarket 15x8s.

So my question is what diameter tires would give me optimal performance (power and mpg) if I am cruising at about 65-70 mph on the highway?

I am leaning towards 255/70/15, a diameter of 29", would this be ideal?
Any input is much appreciated. I am very grateful for the wealth of knowledge from members on these forums, and hope to contribute during my rebuild. Thanks!