Since the transmission will be a late 60's 440 23 spline, that question is sort of already been answered.
11" bellhousing 2892262:
http://www.brewersperformance.com/proddetail.php?prod=BH262
Uses the 4.805 retainer.

Funny side story, though - the previous owner (who did the "conversion" on the car to a 4 speed) had installed a transmission with the smaller input retainer on this same bellhousing - so basically, the only thing centering the input shaft were the transmissions' 4 mounting bolts.
True story, looked funny once I took that one out. :-)