Which heads?

BsB Duster said
It has a brand new set of 275/60/15 drag radials.

Whoa that’s a big tire for that cam and stock stall

Will the B&M flexplate allow me to run the neutral balance 273 convertor?

Yes, that is a zero balance converter or neutral balance.

I can get a stall if it will benefit me that much more, although I need to check the 904 spline count because I think a 1967 904 is different from 68 and up.

I had that exact issue with my wife’s’67 Cuda and the 904 I came with. It had the odd spline count. I found it impossible to get even a replacement converter never mind a performance unit.

So! Down the block from me (Literally) is a converter shop called Pro Torque that custom re stalled and built up the OE converter. Expensive but worth it. Lots of ceiling room. Perhaps that idea is a workable solution for yourself. A capable & Local shop?