/6 questions

Really whatever fits with your spring chassis width, was targeting the most mass production and availability for the least expensive parts availability, which would be the GM based bus platform, but I'm sure D500-D700 with the big Dayton cast iron 5 point wheels would be great and may even be a bolt in. I am reminded of an '89 F600 dump truck at my last job. Could no longer get front end and front disc brake parts, not even from a dealer. Had to red tag the truck after going through a "Friendly" V.S.P. weigh station/checkpoint and I still don't know if they've been able to auction it off yet. This was back in 2013, so always check parts availability on whatever you find. You had mentioned C-60 (which 50 and 70 are similar), C50 would be very close to your present chassis, so I was giving you a heads up where to look. Around here, GM based bus would be the cheapest route, but you may have something completely different available in Yakima there. Just take measurements and keep your mind open when going on the hunt. On a side note, and I don't know how I overlooked it, but we are having quite a few IH/Thomas DT444 powered buses make it to auction now. I had a friend buy a mid eighties GMC 60 series with an 8.2 detroit in it for $2500 at auction. Find an International bus and you'll be keeping it "All International", just updated. Being IH doesn't change truck platforms very often, what was made in the 90's is probably still being produced today.