There are reinforcement pieces on the inside of the body panels. The ones on the tailgate would probably not be to hard to get in there, but the ones in the fender look like they would be a real pain. Your best bet would be a custom bumper with a built on tire carrier (specially if you ever plan to cary a large tire on there). Those can be kind of pricey though so you may be stuck with your inside mount. The stock outside carriers are just not strong enough to carry larger heavier tires and if your truck didn't come stock with one, I wouldn't even put a stock tire on there without the proper internal supports.
In fact, I thought the stock one would be ok on the streets with my 35" spare but I just moved it to the cargo area because I didn't like the way I could see it wobbling around in the rear view mirror. A 35" spare takes up a LOT of space in the cargo area but I live with it for now since I'd really hate to see the skin ripped off my tailgate.