Pre-magnum flexplate on a 46RH?

FatNancy said
The tech said pre-magnum balancing, so I'm pretty sure your first assumption is correct. But would a flexplate like the B&M 10239 work with a 46RH?

I think I misspoke- 518/46RH are 727 based, so you need a B&M 10236. The 10239 is for 904 and it's derivative (A500/42RH) trannys. I apologize- I've had 904s on my mind lately.
Anyway, you've got the word from Johnny M. himself now- good luck!