There is no amount to mill off the head. Tell the machine shop to cc the head and shoot for as high of a compression ratio as you can run on 87 octane fuel with whatever cam you use. Also its better to mill the block vs. The head so that you can retain some possibility of quench.
Al you need for your exhaust is a manifold with a gasket match port job for exhaust. Run 2-1/4" single and you withh be fine.
Unless you run a four barrel. Then do some form of dual manifolds either dutra duals or your own home brewed setup. Use a y-pipe and run 2-1/2" single from there.