Which bellhousing to use?

Can't you swap the 4.80 bearing retainer with the 5.125 piece. I thought that was what I did when i put the OD unit in my 68 roadrunner with stock bellhousing. I think the smaller 3 bolt one is the one that won't swap. The 4.80 and 5.125 shares the same 4 bolt hole pattern. It was a long time ago when I did that.
And I think you can buy those parts new.