Okay, I ordered the Xray 305443 186MM belt and it is a perfect fit.
So, if you want to upgrade to gear diffs, 38T is your only readily available option. The 305432 is the front belt that works. You will need to modify your chassis to have slots/holes under the diffs for clearance and debris relief. The sprockets will still be above the bottom of the deck though, so no worries there.
I'll be racing my new setup this coming weekend.