TC to flexplate issue

[Daddy Bartholomew]

Swapped engines between my 1971 and a 1978 4x4.

1971 beginning setup: 350 cid, 700R4 (originally TH350).

1978 beginning setup: 400 cid, TH350.

400 went into 1971 with no issues, bolted onto 700R4, have been driving it for a week or two with no troubles.

Just dropped the 350 into the 1978 yesterday, bolted onto TH350 pretty easily.

Went to bolt the torque converter to the flexplate today, and discovered the torque converter is jammed tight to the flexplate or the crankshaft. Lookly closely, there appears to be a very narrow gap between the block and the transmission on one side. Turning the flexplate by hand causes the torque converter to rotate.

I haven't run across this issue before. Both engines are for automatic transmissions, they both were for 1970s era TH350s. I didn't remove the flexplates from either engine. As near as I can tell, the nose of the converter is jammed against the crankshaft. The converter is seated against the pump fully, there is space between the converter and the flexplate at the bosses.

Is there a substantial difference in the 4x4 converter? What else might be the problem?