Many of the vendors like Legendary may have stuff for C-Bodies but you need to call and ask. It seams like I'm saying this over and over and over.
Companies like Evans Wiring
http://www.evanswiring.com/ can make a harness with a sample even if not perfect.
Again it may not be listed on their website but they will (most), you just need to call and ask.
I have been working with Detroit Muscle Technology of getting Gaskets and Seals for a 69 Polara/Monaco and some of the parts will work on just about any 69-73 C-Body.
Alan