I thinned a 428 sprocket to 420 recently using a linisher (spelling?). Worked fine and didn't take too long. It's easy enough to get good enough. Only remove material from one side.
Flat top ports are not ideal, check the ESE thread. The exhaust is going to need a curve on the top, so if you keep the same max height you will end up with less exhaust time/area and less blowdown. The width really is on the edge of what's acceptable so you will have to be careful.
Bookmarks