I had run across some info about X-Stream when I Googled some random searches with the info I had to work with, but didn't realize that they, in fact, had perhaps owned this bus. Glad you found what you found... Interesting piece, to say the least. I'd be curious to hear what the former owner had to say in a follow-up.
I want to be clear, though. I don't think this was necessarily a cobbled-together Frankenstein. I think, as with most light/medium-duty trucks, the D-series had 110 / 220 / 330, etc., versions... 1-ton, 2-ton, 3-ton, and so forth, as with American Fords built as F-150/250/350/450/550, GM and Rams (spun off of Dodge as a single division) built as 1500/2500/3500/4500/5500 versions.
It looks to me like a factory school bus built on a 2-or-3-ton D-series frame with some minor customization to the nose. Most buses in those days were simply incomplete chassis (frame, suspension, steering, powertrain, driveline, nose clip, but no cab) sent to a coachwork company such as Blue Bird, Carpenter, Thomas, or Ward, who then installed their own cowl and purpose-built bus body, delivering them as a completed vehicle. They really were nothing more than a logging truck frame with a bus body back then. This practice is still done today for factory-built RVs, even some school buses.
Here is a pic of a '74 D-series rated over 3 ton...
74 D-series 3-ton.png