Sorry, no specific formula to determine literal "best" gears.
But I do the math using differential gear ratio, rear tire diameter, and engine rpm to help choose the most useful transmission gears available from the transmission maker.
I recently did this for a 5spd install on a 1965 GT350.
It was a lot of various inputs to figure the results, but it was not too boring or troubling.
It was worth the investment in time because the car was nicely transformed into a much better performing and driving vehicle.
Be sure to ask the transmission supplier what they have available for transmission gears to be certain you know what you have to work with.
