Small block combo suggestions

Actually I'm running 340 hp manifolds the exhaust system is the hp exhaust from factory in 1967. And yes it's a single exhaust wt muffler and resonater
In 67 no dual exhaust offered.
The gas tank retention strap is in the way on passenger side
The top end of the engine is all six pack stuff.
I'm curios, not to undermine all of the good information in this thread, what makes the 1967 single exhaust with resonator high performance?