they are 2-1/4...i purchased the dynatech header ( weld up) kit and welded them up. they have 4 inch collectors.....they seem to work well...
i also had them thermal coated at a local shop that does coatings...
My vette has 2-3/8 stepped to 2-1/2 with 5 inch collectors..( hawg legs)..lol
Later- Brian