I don’t believe you need to remove that nut. I have never needed to. But it is a PITA to get the belt past the pulley and crossmember. You squeeze it thru by rolling the belt and turning the crank. Watch out for the little pink thingies ( fingers) to not get pinched.
I was incorrect--thanks, Steve--only the later BJ8s have the half-inch belt. OP didn't specify earlier or later BJ8, I think a 3/8" belt wouldn't be much trouble.