89 Mustang rebuild (elect.question)

What is wrong with your harness? How much do you want for it

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.