1973 340, want more power.

If I was drag racing all the time, the 3.91 gears would make sense. But, let's go over again how are you using this car.... You say 99% street but is this street racing? Or just cruising? If cruising, you are probably not going to be happy in the long term putting that amount of $$ into a 3.91 gear set that makes the car noisy and vibrates more at highway speeds, and drops gas mileage a lot. All of the engine operations will now be moved HIGHER in the RPM range, which is opposite of what you seemed to want to do earlier (post #24).

We have been talking about a lot of things but your use of the car keeps falling out of the picture. Bad advice and decisions flows from not clearly staying focused on what you will do with the car.