First thing I would do before I spent a bunch of money chasing my tale with the motor combo is, pull the motor and bring it to a nearby dyno service that’s known for legit numbers and retest the motor.

A fresh set of dyno numbers would verify you have what you think you have, as well as where the peaks are occurring.
I have re-tested several motors where the owners have spent thousands of dollars trying to get the car to go as fast as their dyno sheet said it should....... only to find after re-testing that it didn’t have the power they were told, and the car was actually running just what it should.

You need to test the converter flash.
What the converter “should” be and what it actually is could be very different.

I’m not a converter builder, but if the motor really makes peak tq at 6300, you’re going to want like 6500 stall.
I can’t imagine an 8” converter flashing to 6500 behind a 451, and still be efficient enough to provide an adequate level of lock up going down the track in a 3100lb car(unless you were shifting at like 8200+).
I don’t know what you have, but it sure sounds like a combo that would need a 7” unit to meet that criteria.