How heavy is your car? I think your gearing is OK, Changing to 3.91 would not make much difference. A 29 tall tire would be your best bet.
What converter stall and how much slip?


70 Dart Swinger, 2850 lbs
SB 408, Bullet roller 264/268 @50 .636 SSDS stage 2 Edelbrock heads, 1 7/8 Headmans, 1050 dominator by Dom, 9.867 @ 133