Will,
I would like to know at what RPM you are actually seeing as the stall speed of your torque converter.
I think you may have some torque multiplications effects going on - Ill assume it is a non-locking converter?
500 ft lbs at ~2500 is slightly rediculous. Back when I was in the camaro world we used to see this alot with unrealistically high torque below the converters stall speed due to its viscious coupling torque multiplication. you had to disregard those numbers and look at the charts second torque peak for a realistic evaluation.
I would expect to see a huge torque spike leadig up to your stall speed, and then see it falling off sharply as the converter actually started to engage.
Could you post the graph?