When I did mine last year I bought 12" rims with a 7.5 backspace and left the rear end stock! but my 68 has more room on the wheel wells.

First measure from the front of the brake drum wheel mounting face back and forward to see how much room you have. the look up the tire you want to use and look for the center section with.

Lets say it 13.5" so on a 10" rim it will stick out 1.75" either side of the rim and you want at least 3/4" clearance outside so you need 15" min to fit said tire in! Then so you have to allow 2.5" outside the rim front and back.
so say you have a 10" rim and get a 6.5" backspace from the face of the brake drum you would need 9" back to the inner wheel tube and 6" out to the quarter panel wheel opening. Kinda not shortening rear end set up!

So if you are going to shorten the rear 4" then you would want 2.5" backspacing and have 5" back to the inner tube and 10" out to the wheel well opening.