I used to drive bus (owned a bus company) and have converted one into a show car hauler. It hauled two cars well and had a 370 gas engine it. They can haul far more than they do just for safety reasons. It was an "86 so it had a big split in the two speed rear. It would run 80 miles an hour all day.
I"ve seen it done and they work good for a few reasons. First, the floor in them is strong. If you cut the seats off with a torch and leave a little of the leg there you will have bales that won"t shift much. Also, they have good ground clearance to get into and out of fields. A 1967 will most likely have a 4 speed and a two speed rear axle. That rear axle in low is a stump puller. No lie - you could out walk it in granny gear low axle with your foot burried in the carburetor. Remember, back then most highway speed was about 20 miles an hour lower than now. It won"t be geared high. If it is an automatic, the first in those is pretty darn low. I had one auto and that thing would beat cars away from a stoplight.
Your worst problem will be parking the eyesore when you aren"t using it. I"d love to have one to use that way. I would put chains on it and feed with it through the winter instead of pulling a hay wagon. For the use you will get out of it, a big bus is a no brainer.