I think most people that equip large tires eventually go with an aftermarket rear bumper with an integrated spare carrier or fab one themselves.
If you don't do any off roading, you could probably get away with a larger tire on the factory carrier. The reason people suggest not putting a large tire on the factory carrier is that over time the added weight could push and pull on the passenger rear quarter panel. That and it could pull the portion where it latches onto the tailgate out of the tailgate.
Having said all that, I have a 35" MT tire on my factory carrier and haven't noticed any damage yet, but I honestly haven't put a ton of miles on my Bronco since having it. That and I'm not worried too much about how my Bronco looks.
35" Spare on the carrier.
Bronco I found in the JY where the latch on the tailgate had broken. This is a possibility with running a large spare tire.
In the end, if or when I get a new rear bumper for my 78, I will definitely get a spare carrier built into it. It'd be much sturdier and secure in my opinion.