By 'EFI manifolds', I assume you mean the exhaust manifold, right? No headers? Auto or manual trans? Rear gears? Street or strip?
In any event, this doesn't sound like it's grossly overcarbbed to me. In fact, it sounds like the setup that came on 302 Mustang GT's of the mid-80's, before Ford went to EFI (different engine, I realize, but if anything the Big 6 has a better vacuum signal than the 302). If this setup is in a pickup turning low RPMs, then it might not work all that well right now, but certainly can be made to work. I am no expert on 4150's, and am not familiar with the "Adjust-a-jet", but in my experience, tuning a carb is a lot more than dropping in a bigger or smaller main jet.
I realize this is getting pretty far off your original topic, but if you are having challenges with getting this setup to work right, give us more details and we'll see what we can do to help you. There is a lot of collective and individual experience here, and I am sure we can get you moving in your desired direction.