Thanks David. Prior to this enquiry, I used to think(incorrectly) that a 12.5" rim measured 12.5" flange to flange, but now know that this is actually the number from bead to bead. Since I am thinking about having the rear rims modified, I just don't want to end up with something that doesn't fit exactly correct. Once they are made I own them.
Thanks again guys for the help.