The 6x5.5 or 6x4.5 part is the wheel bolt pattern
The first number is the number of bolt holes in the wheel or the number of studs. The second number is the diameter of the circle formed by the lugs (or bolt holes). These vary by manufacturer, GM, Ford, Toyota etc. For gm trucks, most models any way, they are 6x5.5 or 6x139.7 Same thing though, 5.5 inches or 139.7 mm This is for the half tons anyways. The HD's are 8 lug and are therefore 8 x diameter.
So, for your truck, you would need a 6x5.5 or 6x137.9 bolt pattern. Be it 17x9 or whatever you choose. Also playing into your decision is wheel offset, backspacing, tire brand and actual size, but there is a lot of options covered in this thread of various sizes and combinations that should lead you to a good fitting setup.