I pulled it out again, and it appears to me that the lip on the hub of the TC could be ground down just a bit, eliminating the interference.
It is definitely a junkyard converter - there's an "81" stenciled on it as well as "GM3". It appears to be the converter NAPA lists for a 350/TH400 combination. On the other hand, NAPA doesn't list a 350/TH350 application...
Anyway, can anyone think of a reason why grinding the lip slightly could be a problem?