390 w/open chamber heads

I agree, if you have access to a die grinder and all the safety gear you are better off to match-port the performer manifold about 1-1/4" deep into the runners.

Don't make the manifold ports bigger than the cylinder head ports or you'll just make everything worse than when you started.

Keep in mind though porting IS time consuming, even on aluminium. Expect to spend about an hour on each port to do it properly.