So the shim issue is if you wanna put a different carb on there but it should just fit any old round bing straight up. you do need the lil o-ring in there, same as a stock intake. other carbs you could probably make a dellorto SHA or something fit, but why? bing 15 is the carb for this intake, and is great. i guess if you had a spare SHA around you could save some bucks that way.
user Vlado recommended using the SHA puch intake, and dexcribes making that work here: