NorCal truck has a DIY on how to add some space in the front wheel wells if it's needed:
http://norcaltruck.com/2007-5-2013-chevy-gm-large-fender-mod
Yes - it could be the extra width that rubs. The difference in width between a 265 and a 285 tire is about 3/4 of an inch. Combine that with slightly increased diameter of the 20" tire and it may come **** close to rubbing with the wheels cranked for a turn.
Part of my obsession with the brakes is that I have spent most of my life driving sporty type cars. I like things that go - but I like them to stop too. Given that the truck will be used for hauling and/or towing substantial weight - and out here in NE there are mountains that do have some steep roads going thru them - I'm concerned that brakes that I consider to currently marginal at best - would be outright dangerous if I was in a mountain road situation. I can see the darn truck just not stopping at all.