318 with Edelbrok perfomer intake and 600 cfm 4 barrel, what headers please?

junkyardhero said
does anybody even sell 1.5" primary tube headers for our cars?

anyway, to the question at hand. 1 5/8 and call it done. best fit chassis and motor wise, broadest selection of mfgrs

I can't recall if the old early A-Body Dougs were 1.5 off the head. Mapes has them on his Goldfish. Those were something like three sections on one side.
That's the only off-the shelf ones that come to mind.
1 5/8 is far more common that's for sure.