I have a 1975 GMC C15 step side. It came with factory dual tanks and has a factory crossover tube. The cross over tube goes to the switching valve. I am rebuilding the truck and kept the dual tanks since I am staying carbureted for now. Its actually an easy restoration and reasonable cost...
Lots of great info from all the other guys. Honestly you need to add the question of how your going to drive the truck. Highway driving where the OD would be beneficial or short distance driving where the higher RPM's aren't an issue. You could also always change the rear end gearing to lower...