I'm all for using what ya got till ya can get what ya need.
You "could" do all the changes and used the 1407 "because you have one" and disconnect the secondaries.
Thus having a 2bbl till you can find a used 390/500. They come up on all the "Bodies sites" every now and then.
Also I remember Doug Dutra ran a 1407 on his street slant for a bit. He was at a Mopars at the Strip event and had carb problems and picked up a 1407 at the swap meet stuck it on and went drag racing. Just a thought.