The shoes ol' Bubba's wearing now are 245mm ( 9.645 inches ) wide, the 265's are 265mm ( 10.433 inches ), a difference of 20mm ( .788 inches or roughly 3/4 inch ). I personally don't like to see duals any closer together than 3/4 of an inch, but I've noticed several GM's as yours are.
They make some very good steel spacers with studs pressed in, that come in a couple of thickness'. In between the studs are holes for the OEM studs. After the inside tire is put on, the spacer is set in place, and the lug nuts are installed and torqued to specs. Once that's done, the outer wheel is installed on the spacer plate studs, and the lugnuts torqued to specs. If you have 1/2 inch of clearance now with the 245's, and you want about an inch with 265's you will need a spacer about 1 1/4 inches thick.
I have seen other spacers that just have holes for the OEM studs, and they probably work okay, but the thickness is limited by the length of the OEM studs. If yoy have at least 1 1/4 inches of threads protruding from the lug nuts now, they might work for you, but the other style is better.
Hope that is some help, and not too long winded.
On Edit - Sorry Dave, I didn't really answer your question. We don't have any trucks in our fleet running spacers at the present, but we have used them in the past with ecxellent results. We do have one Dodge that could use them now, as I put some wider tires on it a few days ago.