Thanks. Not here to boast but just wanted to help others out. I couldn't find a whole lot of info/pics out there but what I did find on here helped in my decision.
As far as width, I think a wider tire would look better, but I was afraid it would rub. I came from a 2016 2500HD (gas) that had 35x12.5r18 Firestone MTs (Gen 1) on stock rims and they would rub pretty bad at full lock. I had it leveled and CST UCAs on it. It would rub the UCAs and the sway bar at full lock pretty good. It had already wore the finish off both. It got kind of old. If you can live with it it's manageable but I just didn't want to deal with it on a brand new truck. I think you could fit a true 35" tire on the Trail boss stock. I definitely have plenty of room in the rear. The front may get a bit tight due to turning, but I think it is do-able. I honestly don't know if or how much wider you could go and not rub. To me, it wasn't worth the risk. I tried to take different angles to paint the best picture to let others decide. 35x12.5 is the max you're going to get without a full blown lift IMHO. But tires vary so much from different styles, manufacturers, tread patterns, etc. Whereas one tire might fit that doesn't have an aggressive sidewall, one that does have an aggressive sidewall may rub. It's more of a plug-n-play.
All I can tell you for sure is this size & tire fits on a bone stock Trail Boss with no rub whatsoever. And bottom line you gotta make you happy.