Was the 396 as in .030 over 390 or as in destroked 427 like they ran in Nascar for a while?Do the cross under pipes give a significant power increase or is it mainly for fitment?
If you mean the picture I posted, nothing fancy, just 30-over 390. The forged pistons were the big ticket item at $20 - EACH!! $160 for 8 pistons, you could darn near buy a running car for that. The headers had just come with another 390 Gal, rust-free, we bought the car & free headers for $500 or something.
They're NOT 180-degree headers, there's still a pair that fires at 90 degrees on each side, so they dont make that awful import-style buzzy sound.
Probably the routing was an effort to make them near equal length so tuned for a peaky engine or one that's kept near peak revs.
From the header modeling I've done you probably would get a higher HP peak right there, but you'd pay for it elsewhere in the curve.