a Body 4speed

It should fit any A-body but, if it is a 1966 transmission it's a bit of an oddity. 1966 A-body 4-speeds were a 1 year only deal that used a 26-spline output shaft (same as a 904). From 1967 on, they used a 30-spline output shaft (same as a 727).

http://www.brewersperformance.com/history.htm

Also, I seem to recall reading somewhere that a 904 slip yoke will not work correctly because of the output bearing and seal setup. I can't seem to find that off hand though.

Perhaps the seller can count the output shaft splines or post a picture of the shaft?