Magnum EFI Return or Returnless? & Vacuum Regulator Question

I guess I'm thinki

That's something I hadn't thought of! So the vacuum reference regulator would go to my adjustable regulator vacuum input?

No that's it. Just that rail. There would be no need for any other regulator. You'd use the stock fuel regulator mounted in that stock magnum fuel rail. That is all.
Its already preset to the stock fuel pressure

You need that rail, a fuel filter and fuel lines(fittings, ect). NOTHING else is needed to regulate fuel pressure.