Couple things I'd ask about before going farther. What classes would the streached frame and diesel be okay in ? Do you have to use a GT rear or not ?
If it's okay
I'd scrap the rear in it and find a 71/4 Chrysler from a Panzer and bolt that in for the rear. Drop the motor in and use a trans from an Economy and a clutch from either Lakota or Vogel. and the mounts from a cub. As for the gearing if you overlap the rear input and the shaft from the cluth , you can run sprockets with 60 chain and adjust easily.That way your using all garden tractor parts also.
Well this could work but to a point. And it seems like alot of mess to get in to not saying its a bad idea or anything but its alot of work to do for a rearend. If you were to find an economy tranny im sure there on the pricey side and with all the gearing and such. And for someone who would be new to this I would just recommend sticking with the cub rearend for knowing that they can holdup and knowing that parts are easy to get to. But the idea is welcomed!