[G.M.]

You can mark the crankshaft and mark a tire at the floor. Put the transmission in high gear and push the car by hand. One complete turn of the wheel and watch the crank shaft counting the number of turns. A little over 4 turns= 4:11, 3 3/4=3:78,
3 1/2=3:54. In my opinion from several hundred thousand mile on old Fords over the past 30 years mostly in 39's the best rear is a 3:78 with a Columbia 2 speed which is a 28% over drive. At 65 MPH the engine sounds like 45. My second choice would be a 3:54. G.M.