727 transmision problem

If you didn't get the conv hub all the way into the pump gear, it may have sheared the keys on the pump gear. At high rpm friction might be allowing the pump gear to turn enough to push some fluid to get the car to move.

Also, last year I built a 46re for a guy in a 99 Ram. He installed the converter on the flexplate improperly and it completely wiped out the converter and pump. Siezed the hub bearing onto the hub and proceeded to waller out the bearing seat as the converter was not positioned straight on the flexplate. His did exactly what you're talking about to the T.