Another factor is the exhaust port, this is what limits flow, not the manifold or pipe. If the exhaust can not get out of the head, pipe size doesn't make any difference.
Joe
No doubt it was a lot more fun discovering this at the drag strip as opposed to making a flow bench adapter plate, documenting effect of alterations, and digitizing/dissecting the head.
There was an inliner who made nice split manifolds and whose work I tried to emulate but stopped some time ago. I believe this was because only about one third of those that exist are suitable for welding and you would soon run out of material. Heavy use (overheating) makes the iron brittle and exposure to the elements (rust) prevents success. I may do a few locally if provided with good cores but other than that it's not really feasible as much as I think this is the best method and would like to provide this service to stovebolters. I once hinted at providing the machined flange, elbow, and instructions in an old thread where all you needed was an old fart welder but all I heard was crickets
Also, The 250/292 factory exhaust manifolds are not good candidates for dual modification due to their design but that's moot as far as 235/261 flow. Frank McGurk's HRM article documents the difference the dual Corvette exhaust manifold showed over stock which IIRC is was significant especially when combined with other modifications.
