Join Date: Nov 2014
Location: Marshfield, MA, USA
Sounds to me like that might be your problem... but if it is, you might need a new flexplate.
Years ago I'd purchased a 5.0 (which I never used). The seller got rid of the engine because there was a vibration he couldn't resolve. I removed the flexplate and discovered it was bent. That drain plug might be the reason it was. The seller may have installed the converter incorrectly, removed it and re-installed it, but the plate was toast after the first try.
It made a nice clock though.
69 Convertible, 351W, T5, E-Brock Performer 351-W intake, Performer Plus cam, Holley 4160, power assist, dual exhaust, 4-wheel manual drums.