If you are running water injected headers, I would use stainless braided line on what will be closest to the pipes. I melted a hole in one of my lines and it took me about 3 trips to figure out where all the water was coming from. I will be using them when I re-plumb everything. For anything that isn't exposed to such heat, 5/8 inch water heater hose works just fine. Just my .02