It sounds like a common problem. On a JD 305, the hydraulic pump shaft runs off the front of the motor and has the pump in front of the radiator.
I took the old belt to the Auto parts house and got 2 belts I thought were about the right size, then returned the one I did not use.
I had to remove the bolts for the hyd. pump and pry it just far enough out of the way to work the belt between were the shaft splines slipped apart. I did not remove any hyd. lines or hoses, there was just enough flex to get stuff worked apart. Then was able to get everything put back together.