My experience is with mostly stock hardware, and on the exhaust side of things, I am wondering what people run.

Stock, the 75-1980 20R has a 4 into 1 cast iron manifold, and no cat (to my knowledge)

Stock, the 1981-1982ish have a 4 into 1 cast iron manifold, with a dual-pipe flange (which 'Y's downstream), and catalytic converters...

Stock, the 1982-198x have a 4 into 2 cast iron manifold, with the dual pipe flange.

The earlier manifolds between the 20R and early 22R interchange, and the later manifolds have a slightly different bolt pattern - 2 holes in the center, versus the single one on the earlier.

So, my question: If I run a split cast-iron manifold, and separate ~1 3/4" (to match the factory pre-'Y') pipes, dual small mufflers, and ~ 2 1/2" outlet (underneath or stack), what sort of power increase/decrease, economy, or sound I might expect, and whether anyone here runs something similar...

I'm also toying with the idea of fabbing a 4 into 2 header, with similar sized piping, mufflers, and outlet (conventional or stack)

Also, I'm sort of curious as to what type of set-ups you all are running...