What is the loaded diameter of the tires you would be using?
Change the transmission from an original 3-speed to an original 4-speed will have no impact on speed-rpm. Both are 1:1 in top gear.
I can cruise easily at 65 with Patrick's 3.55 rear-gears and 31.5" radial tires. 2450 rpm at 65 and 2850 rpm at 75. I feel safe cruising at 65 or less.
