Presently, I have to theorize (risky, I know) that it is a correctly matched forged crank combo, simply because it ran smoothly all the way to 6k rpm with the 4-speed behind it before. The options I see are, 1) I have a bad torque converter, or 2) Maybe something about running it without the yoke/driveshaft installed is causing it?
I'm going to get the date code off the block and see if it jives with my hunch about the paint. That would be weird. What year started the cast crank 340s?