What engine ,what cam,
The ratios in this box are not the best for a hi-reving SBM.
It's not really a 4spd, but rather a 3plus1. And the o/d is somewhat fragile.
I got a buddy who got a OD from me and put it in his 69 GTX 500 plus HP. He beats on it all the time and it's doing fine. Its been in the car for 2 yr.
