Any of the metal plumbing fittings are OK for Diesel
You might prefer Bronze or Brass. If your tank is stainless or aluminum you could use that, and if steel you could use galvanized iron (not normally recommended on boats), and even that used inside and under-deck will work OK for many many years, even if it rusts somewhat on the outside. I had galvanized fittings on a home fuel oil tank for over 25 years within 15' of my sea wall (salt water). They (any metal) will also all work on a fiberglass tank.If you try to bend 1&1/2" fuel hose, remember you will probably (if you are coming off the top of the tank) have to make 2 turns or an S curve. How much room that would take, depends on the horizontal distance between the tank out let and the deck fitting.Good LuckJoe S