The o.d. bearing retainer can be swapped for one that will

a) fit the bearing
b) fit the input shaft (18 or 23 spline)
c) fit the bellhousing as is
d) bolt to the case

This is a hell of a lot easier and likely cheaper than buying and boring a bellhousing. Check Brewers or Passons for the one you need.

I'm thinking this one from Brewers:

BR-696 BEARING RETAINER 4.805" DIAMETER 308 BEARING 23 SPLINE NEW CAST IRON Details

New cast iron bearing retainer for 1970-up B/E-body 23 spline transmissions with 308 front bearing, 4.805" outside diameter, 4.16" bolt circle, replaces cast #C99369 original retainer. Gasket and seal included
Price: $79.95