89 Mustang rebuild (elect.question)

Unless the whole harness is destroyed and the wires are all melted together from an engine fire or something, then ... why bother? Just re-route them as you see fit, then re-wrap them with tape and/or electrical conduit. Done.

That being said, I've seen complete EFI wiring harnesses for sale on eBay from guys who've gone to carb setups. I think Painless Wiring has a setup, too, but theirs is more of a universal deal rather than an OEM type of arrangement - y'know, for cases where you're swapping an EFI 302 into something non-factory like a Jeep or an old 60's Mustang or whatever. There are places that will custom-manufacture harnesses for you, too, but be ready to pay out the yin-yang for that sort of thing.