Well CRAP!
View attachment 397759
This old transmission input housing has part number C94796.
That's the smaller 4.354" outside diameter/3.70" bolt circle, originally installed in 64-67 b-bodies, 64-75 a-bodies and 70-up small block e-bodies.
It's the WRONG DAMN TRANSMISSION!
Man, I hope to heck the hole in the bellhousing is bigger than that so I can replace this thing with a proper big block transmission!
I'll get under there and measure it tomorrow....
dammit.
There is hope, though. It looks like there's an imprint of the bellhousing hole on the front of this transmission and it sure looks bigger if that's what I'm seeing.