In my old Duster with 3.23 gears in back and a 727 with a stock type 340 converter I used to turn around 3200 rpm at 70 mph with 26 in. diameter tires. Never seemed to hurt a thing. My 'Cuda with 3.55 gears and .68 overdrive [ final drive is 2.41 ] and roughly the same diameter tires is just under 2200 at 70 mph. Easier on the ears, and probably longer engine life. Both cars used a similar .030 360.