You don't need back pressure with a 3800. you never need or want it on a forced induction engine.
anyway and the few motors that actually benefit from back pressure usually come with small intake and exhaust manifolds and no better options for them.
From what i remember being told by my ancient mechanic:
The enlarging of the exhaust on those small old engines tends to create turbulence in the exhaust and slow the flow
depriving the engine of high rpm horsepower and some torque.
now days with ecms and such let it flow as fast as you can.
[This message has been edited by Strappado (edited 02-14-2018).]