The op can confirm or deny this, but here's how I see it played out;
Build car/motor, buy a new 5000 stall converter for the combination, go to the track, car won't leave. Calls a few people, a few people see the car run..... the consencus is the converter is wrong for the combo.
Get new 5800 stall converter built, car runs the same.

Since he says the car pulls hard after shifting into high, my feeling is the trans is bound up in low gear.