We have a 21 ft hopper, I have used a 6x41 Westfield that worked fine for unloading, but it has a mounted engine.  I think 35 or 40 ft. would be fine.

I can't think how a standard pto drive would work and have room for a tractor, unless you design a belt drive or something to relocate the pto shaft.

Would electric drive be an option?  Or hydraulic drive?  I think you can get engine drives for 8" augers also.  I just checked Westfield's website and they show all these options, including pto belt drive.