2005 1500Chevy Truck Tire Help

I need some help on buying new tires my factory tires are P245/70R17 they have about 42200kw on them & starting to look weak. I wanted to replace them with a good Michleum tire, I had one tire dealership tell me I had to go back with the same size tire or it would rub the fender welds are have problems when I trun, I wanted to go with a Michleum XLT P265/70R17 he said that was  pushing it cause the tire is a bit higher & wider. I think this is BS has ant of you guy had this problem. HELP