18lbs worked best for me in my 3800 lb race weight 63 dodge and also my 4000 lb 65 Coronet wagon. Both ran 275/60 MT drag radials. My 63's 60 ft was 1.63-1.66 at 11.5-11.7. The wagon went 1.48's at 11.0's. Not sure about the 4 speed factor though. You may have to mess with it.