can't answer your first, but can sort of help with the second.
your factory harness doesn't really matter, because you aren't really running through it for anything but power and ground.
the only real catch might be did you upgrade the alt for power.
adding efi pump(s) might start pushing your limits if you are still running a 60 amp alt depending on what else you have drawing power.

90 percent of the wiring in my 65 coronet is the original harness. But I spliced in relays for a lot of aftermarket stuff along with extra wiring for other things rather than power it through the original harness.