Hallet, since this was one of the business' that I've owned I guess I can tell you BUT you ain't gonna like it.
First of all, you should know that you can get a manifold polished for $100. to $300, depending on type of manifold. Second a dremel won't get it, not enough power. My guys used die grinders rated a 1hp for the initial grinding(smoothing), For one this powerful, you'll pay at least $400.(air). For a compressor, try 5 to 6 hp and atleast 60 to 80 gal cap. Use 1/4in mandrals and sanding rolls of 100 grit to start and then go to 180, then 240,then320 or 400. When manifold is smooth enough to suite you then go to polishing rolls,( I like yellow but some guys use the white ones), 3/8 thick by 1 1/2 in long. You can polish everything, aluminum and stainless steel with the Green compound.
Remember, the more you grind and smooth the manifold, the less you'll have to polish which results in a lot less time spent and alot less mess. YOU ARE GOING TO GET DIRTY. If it sounds like I'm trying to dissuede you from doing this, I AM. If you have any questions you can E-mail me.
Michael
the dog