Find a good 8" and toss that 10" no doubt its slowing you down.
Putting a car on the transbrake and checking the stall is not the way to check the stall nor is it the true stall. In order to find the true stall you will need to find what we call launch fallback stall and after shift recovery stall these 2 will be different and typically the fall back stall is the one you want to nail RPM wise. I can tell you your fall back is prob far from 5100
You are right on it should run mid 10's ..