The most overlooked part of the exhaust system are mufflers. It takes 2.2 cfm of flow per hp for a no loss system, and that is part of the equation for zero loss. A 2 cfm per hp system costs you a 2 percent power loss, and the loss goes up fast from there. There is a very good chapter in David Vizards' book How to build Horsepower, new edition. To make the motor act like it has open exhaust, a termination chamber the same cubic inch as the cylinders(for a motor of 446 cubes, 223 cubic inch chamber right after each collector, no reducer, bell shaped exit) then big enough pipes and mufflers to match the power. For mufflers, to get addiquet flow you might have to use a larger entry and exit and reducers to get enough flow, like 3 inch on each end of the muffler, 2.5 inch pipes in and out.