Daniel,
The belt will probably slip under heavy load. When I bought my tiller, it was attached to a Red CCC 782 with a blown KT17 (Imaging that). The 782 came with the tractor, #2 Tiller and mule drive, and a 44C deck and mule drive / undercarriage. I mounted the gearbox onto my 1450 straight off the 782, bought a 1/2" tiller belt, and installed the 782 mule drive. 2 things with this, 1. The 782 Mule drive makes the distance longer between the rear pulley and the PTO Pulley. On the wideframes, the bar that clips into the front quick disconnect hooks, is the same bar the pulley pivots mount to. On the 782, there are 2 pins welded in, about 1" behind the bar the pulley pivots mount to, so this pushes the mule drive pulleys toward the front of the tractor. Also, the 5/8" pulleys are a larger diameter than the 1/2" pulleys on the mule drives. So, I temporarily used the deck mule drive, so the distance was correct, but it has the angled pulleys, so that is not ideal. I had the 1/2" belt installed, and had it running to the 5/8" pulley on the tiller gearbox. Anytime I started to get a decent load on the tiller, instead of it pulling the engine down a bit, and making it bark, the belt would slip and the tiller tines would stop. In light grass, you may be ok, but if there is any force on the blades, I think it would slip. What I have ended up doing it I am using the mule drive and gearbox off Dad's 149 and everything work really well!!!