Its not about back pressure. Back pressure is restriction. It is about scavenging
Scavenging is about inertia. The exhaust charge is accelerating out of the cylinder, and into the exhaust. After the exhaust valve closes, the exhaust is still racing down the header and toward the tail pipe. At a certain point during its journey, the exhaust charge starts to pull a vacuum on the header and immediate exhaust pipe. This is the ideal time for the exhaust valve to open again. The vacuum from the previous stroke should help "suck" the cylinder free of exhaust(remember, we are talking about engines here).
The same goes for intake manifolds, except the opposite. Instead of a vacuum being formed, a mixture of compressed(at least more than atmospheric pressure) air/fuel form right at the head, ready for the intake valve to open. RPM is critical for these events to line up.
The length and diameter of exhaust and intake manifolds determine the RPM of optimum efficacy.
Late 90s Ford engines (the 4.2LV6 in the F-Series trucks comes to mind) actually used a butterfly in the intake manifold to effectively double the length of the intake manifold runners to make the engine perform better across the powerband. The 4.2L engine ended up being junk, but the theory makes sense.
All I am getting at is... be careful of exhaust mods. You may actually free up restriction, but reduce overall power. Almost all of these after market exhaust systems have been developed by PHDs and dyno tested. I think you would be much better off buying an off the shelf exhaust system.