Here is a story:
I took the TH400 out of my 78 K20 and put it in my K5. Years later, I pulled it and took it for rebuild. The rebuilder took one look at it and said, "you'll need a new converter". Why, I ask. "Because your centering washer is gone. Chevy welds a washer to the front of the TH400 torque converter and that is what centers it in the crank. A TH350 simply has a snout that aligns in the crank bore. Look in the back of your crank when you get home". Sure enough, the washer was stuck in there.
I never noticed a vibration before that, but with my driveshaft vibrations, that doesn't mean it wasn't there.
Also I am no expert on AMC TH400... but my guess would be they are the same way.
1983 CJ7, 4.0ho, NV3550, D300 4:1, Fr D44 Detroit, Rr D44 OX, 4.56, 35" SSTs on beadlocks
1967 CJ5, 231,T18,D18,4.88s,Warn OD, 11" drums, locked/locked
1979 K5, 454,TH400, NP205, D44, 14b, 35" BFGMTs