Another cheap way to pick up some power is to port the stock Series II manifolds. Well only the front one, the rear tubular cant be improved because of the way it'* designed. But the front one that is cast can be ported out (to port the floor you have to add filler material, ie welding on the outside on the bottom of the port so as to not go through, its very thing.) Also open up the roof (where most of the exhaust gas is and work it about 1-1.5 inches in and should be good. Also clean up the end that connects to the crossover, helps the transition as well (clean up the casting flash, etc). If you do this, the front manifold outflows the rear tubular by about 10-15% when we tested it on our flowbench. Hope that helps.
ADTR 1999 Black Bonneville SE
2003.5 Yellow Mazdaspeed Protege
2003 Azure Blue Mach 1 Mustang