I wire a lot of cars and I think you are way over thinking this deal. Wiring a car from scratch is actually pretty easy if you have an understanding of how things work, have some patience and mostly have a PLAN.

For relays and stuff, MANY companies have very nice relay centers already set up and you just terminate from there. I personally use the ones from "Leash" electronics. You can order with any mix of ground or hot relay outputs if desired. It is potted, fused and ready to go.

For a ground, you can do your own "floating" ground setup. Take a strip of fairly thick copper, about an inch wide and six inches long. Put several 3/8 and 5/16 holes in it. These will be your ground lugs. Now mount the strip to the chassis on rubber isolators. The ground cable comes from battery to strip and all items ground to strip. A "to the chassis" ground system is actually one of the poorest setups you can have, yet people continue to do it and wonder why they have wiring issues.

I wire most everything with the "floating" setup I described above, but especially on EFI cars, where clean grounds and RFI noise are big concerns

This is a dragster bulkhead I wired a couple years ago with a nice Painless relay and fuse center. We spread the stuff out over the bulkhead, that in case of an issue it would be easy to look at and diagnose

Monte