Here,s a quick way I did mine(narrowing)for 15x10inch rims.
O.k..stock a-body width is 52 5/8 inches flange to flange.I took a C-body housing and cut to 46 5/8.Basically 3 inches off each side.To run a 10inch rim with minitubs you need 7.5 backspace with a stock A-body rear.So I subtracted the 3 inches from each side of A-body width,allowing me to run a 10 inch rim with 4.5 inch backspace.This isn,t the way everybody does it,but it worked for me and had enough room for a 15x12 rim(4.5 b.spacing) with a 29 inch tire.Hope this helps?LOL View attachment Racing the 66 Dart 024.jpg