Engine locking up??

I am just finishing install of a 5.2 magnum engine using my old 904 transmission. First time turning the key and something locks up. I put a ratchet on the crank pulley nut and the engine will turn backwards about 1 turn and lock! then turn forward about 1 turn and lock!

I took the starter back out to see if maybe that was a problem. Nope.

If I didn't get the torque converter seated right, could that be the problem? The TC bolts went in with no problem.

If that is the problem, I gotta pull the engine out far enough to reseat the TC. Right?