I live in a county in CO that as long as the vehicle physically exits for a VIN check and has insurance, it's able to be registered. Not terribly worried on that part for my use although definitely aware that it could potentially impact resale. The rest of CO is getting a little tighter on that sort of thing but so far diesels just need to pass an opacity test.
And duly noted on the rust. That's the kind of info I was looking for so thank you.
Also - 6.7 Cummins, not Duramax.