Stock header down-tubes outer diameter: ~34.5 mm. Inner diameter: ~32.5 mm.
Muzzy header OD: 1.5" (38.1 mm) ID: ~36 mm
Stock header pipes protrude 12 mm inside the gasket seat, but could be 15 mm or more to be closer to edge of exhaust port outlets.
There's room for header pipes of up to near 40 mm OD, but an ID of about 34 mm would be a pretty good size for the exhaust port outlets which I smoothed out.
Stock header collector pipe is 42 mm OD / 40 mm ID. My Muzzy header's collector is 2" (50.8 mm) OD, and about 48.8 mm ID.
Arrow's header has a nice design with the down-pipes' OD starting at close to stock OD, tapering to a slightly larger OD almost halfway down.
Its collector size is matched to stock 40 mm ID, as its slip-on fits stock header collector.
My Muzzy header is the only one that has a larger collector on which stock or aftermarket mufflers don't fit.
Pics show stock header, Arrow header, before & after exhaust port outlet: