Yes it will work, I use 13mm carbs in the K&B 13.5mm cases for years. Wrap the new carb
spigot with teflon tape, then cram it down tight, and tighten the screws to hold it. I know it
sounds kinda Mickey Mouse, but it works good.

The cool thing is....if you crash the plane, the carb pulls out without busting the case.

FBD.