High end 4-2-1 headers seem to follow this general formula:
Primary id that matches the port area. One step in size (so 1/8") anywhere from 8-12". Primaries seem to end up in the neighborhood of 18-24" (obviously depending). Secondaries are two steps larger than the last primary diameter (so 1/4" step up). Choke merge collector, then they either run back dual, x pipe, or into a single exit. Single exit seems to work very well for Calvin.
It all feeds into a single 3" tailpipe.I would not be afraid at all of using 1.625 at all off an even larger port area. I do it all the time for Cobra Automotive and their nostalgia mustangs and cobras. 1.63@8-1.75@10, 2" secondary for 12-14", (not a typo) and a 2.25" choke at the final collector. 620hp no problem and all sorts of bottom end. Especially for such a small motor and road racing
Incar here:
I thought the NASCAR stuff is about 1.88-1.91od off the head, 2"od step, 2.25 secondary... X pipes because one pipe would be too big to fit under the car (plus aero, NASCAR mandating how the exit can be, etc.).
However, looking at eBay, there are several set ups with steps in the secondary just before the final Y. I have my guesses on what that is about...