Go with the single 260. Adding duals will add obviously the alt. then brackets, then a parallel harness. Doubt you get a good discount on those parts so then the 260 is cheaper and then you don't have to alts in your way down the road when servicing the truck.
'17 L5P Silverado 2500HD, CC/SB, LTZ
'06 LBZ Silverado 2500HD, CC/SB, LT2 SOLD
Never left me walking!!!!