I bought the wrong transmission for my 66.

So this is from Brewers. Same casting number on my tailshaft.

Extension housing casting number C-99539 (with dual shifter pads, BOTH GOOD) for 1970-up B/E-body (also used on Dodge truck A833 overdrive 1976-1987) 18/23 spline input transmissions with 30 spline output/308 bearing. Baked and shot blasted. Uses SR229 bearing snap ring, NOT INCLUDED. (See EH539RO for extension housing, for use on E-body rear shifter location ONLY, $179.95)

I'm pretty sure my transmission is from a dodge truck 76-87 23 spline input, 30 out. And because the rear shift mount bosses have not been drilled, I bet this is not a B/E trans, but is out of a truck. 10-4. But!!!! I wonder if the B/E 4 speed will bolt right in due to the tailshaft housing casting numbers being the same.
As long as the trans mount location is the same? maybe. Great info on this site btw. Off to bed, have to wake at 3:15.