i used a bench grinder and several different files to make it smooth as hell. Go SLOW. I literally had to do it over 5-6 times taking small amounts off and checking to see if it fits.
Loosen the clamp screw on the carb so the metal can bend outwards. You should be able to shove the intake in there with some force. You do not want it to be too loose or else I bet you'll get air leaks.
If you do end up taking too much off, just use like fucking electrical tape around the end to make up the difference. The rubbery texture should provide a great sealing wall.