Did you leave the transmission in when installing the engine?

After cinching up the bellhousing bolts, you should be able to turn the torque convertor independently of the flexplate before you bolt the TC to the flexplate. If not you've found your problem: the TC wasn't fully inserted into the front pump. You should have about 1/4" clearance between the TC and the flexplate.

If you have tightened the bolts and have been turning the engine there is a possibility that you have damaged the front pump on the transmission.

You should be able to tell which way to install the flexplate by looking at it. The crank side should have a faint circle from the crank flange and the other should show marks where the bolts smeared the metal as they were tightened.