8 3/4 ring gear change for optimum low end performance

Brake stall or flash stall?
Ima guessing flash-stall, and didn't you say the convertor had an HS sticker?.
that sucks
that really sucks.
Is the lock-up engaged or mechanically stuck, I wonder.

I have a 367LA in my 68 Barracuda with 3.55s.
At one time , maybe I mentioned this, I had a 223/110 cam in her, and she spun two gears all the way thru with no help from a TC cuz she's an A833 4-speed,lol. That's 70 mph on 245/60-14s. The 295/50-15s spun nearly as far.
This combo went 12.9 in the qtr at 3650 pounds, which translates to about 330hp. That's pretty close to your combo right? Well except your car is way lighter.
So what's the scoop,how's your convertor doing with rpm.