Hi Jordan, you've definitely done your homework because you've described what is (for many) the ideal bus. That, unfortunately, makes the hunt harder...because others are looking for that same combination of features. Since you're open to buses up to 40 feet long, though, that opens up the options for you. It seems many only want 30-footers.
Where are you located and/or are you willing to travel for the right bus?
Here's one current dognose option, not with a Cummins but with the DT466E...also a good engine...but it doesn't specify which model of transmission. It might be work asking them, though. It's in NM, so probably rust-free. https://www.publicsurplus.com/sms/au...ew?auc=2708034
Is that along the lines of what you're looking for?