From what I've learned by my recent crash course as far as ceiling heights go, and PLEASE correct me if I'm wrong!!!
Bluebirds are hard to find in a 78" ceiling...
Thomas's are easy to find with a 78" ceiling. ONE way to tell is the hump just aft of the drivers section. If it goes up 78", down 73".
Another way is pax window belt is set higher than the drivers window.
The AmTran/IC RE 300 Genesis
Not in that order but will send you down the right trail,lol. (Name and manufacturer changed a few times) ALL have 78" headroom and are arguably the best looking cab over skoolie.
Yes 600k+ is a bunch. Less mileages are easy to find also. Plus I don't want to rely on something that high mileage. (I WAS looking at transits with typically 500K at retirement)
It has taken me three+ months to find mine. I lost some bids that now I'm glad I did!!!
Good luck and keep us posted. And be patient.