For what’s it worth I’ll add my expertise lol
my 340 W2 engine had a set of 1-5/8”LA hacked up banged up cheap as hell headers. I ran high 9’s with the wrong headers. They worked.
since then I build a set of fender exit 1-3/4” stepped to 1-7/8” W2 headers. So far I’ve picked up 1 mph.
Years ago a friend of mine spent a lot of time playing with headers on his car. He learnt quite a bit about it and shared it with me. As you have found, it can be quite interesting..