Priobe You need a small amount of back pressure. If not you will burn out the exhaust valves. The ideal number is between 1-2 psi under WOT. 3-5 is OK and will help with drivability. Anything more and you are killing HP and torque. If you are running 500 cubes, you only need the system to handle 250 cubes, a 4 cylinder. I have a race set of pipes and street set that came with the car. I am loosing 34 HP and 6,000 rpms with street pipes but still have my hearing on the left side.
Car maken 2,000 HP are running 4" exhaust pipes. Step headers also help make more power. 3" in and out should be good for your car. The muffler design inside needs to be like an old glass pack like a cherry bomb and straight through. Rick L.