You can machine the entire cavity out leaving only the outside walls. This will leave you with two holes in the bottom that are very simple to braze shut, this gives you enough area to adapt even a four barrel and it will allow you to round the entry to the 1-2,5-6 runners. Then intake can be bolted back to a stock exhaust manifold or used with headers. I have used a belt sander to get a flat surface to bolt an alum adapter plate I make from 1 1/2 or 2 inch thick alum. Using the thicker alum block allows you to drill passages around the outside for additional water heat if you drive in very cold weather. This project can be done with a belt sander and a drill press and a few carbide burrs if you don't have a mill.
Been there, Done that, Hope to live long enough to do it again.