440 duster. Need engine build advice

I would weld up the plenum opening (right under the carburetor) and reshape it smaller like this.

http://www.summitracing.com/parts/edl-2954/overview/

Just get a 4150 carburetor to manifold gasket and lay it on top of the M1 and you will see what I mean.

But, billyb, it isn't neccessary for your build. Only under the most extreme racing circumstances when you must use the single plane M1 should you do it. I have tested the stock M1 with a spacer on it and it works fine.