For those of you who have updated your rear end but are using the stock front parking brake assemblies from the cab back here is a great resource:
Madison Power Systems
1727 E. Auburn Road
Rochester Hills, MI 48307
248-8582-2055
Jim was my contact (and the guy who made them.)
jim @ madisonpowersystems.com [spaces need removed]
Purests don't get angry.. He took me from stock 47 jbar at the "y" to a 69 GTO rear end. He just needed some good measurements and a pic or two to confirm approx. distances. Didn't know them from a cake of soap but they did me well. Most guys who made cables didn't want to be bothered with a "project". Price was right as well (~$120 for pair).
Here's an interesting site www.helmarparts.com for various cable types and lengths. My 71F350 had missing cables and after measuring 2-3 times, I ordered 3 cables from them and fabbed it up to work. Good quality parts and reasonable prices. They are for forklifts etc.