If you are going to run strip any at all, I would stay with either the four speed stick or TCI makes one tough automatic for that car. I use to have a 70 Mach with 351W and automatic trans that would smoke a 69 375hp chevy with no problem. Bottom line it is up to your personal choice.
Then personally I would go with the TCI Street-Fighter with around 2500 stall torque converter or a good beefy 4 speed. It all depends on you! Do you want to shift all the time or only when you choose. The TCI will shift out just like a 3 speed.