The 2-1/8" shouldn't be too big. Below is a picture of a wide 2-1/4" top cut for a 2.02" valve. It fits inside of a line inscribed around the combustion chamber using a 4.080" gasket.
Well, I guess maybe I should have posted this thread BEFORE ordering the kit rather than after, huh? I guess I can blend the top cut out into the chamber if I run out of gutter diameter.