If the truck has been sitting up for a while I would disconnect the line from the tank and look at what comes out. If it is clear and clean go ahead and drain it and add 5 gallons to it. It should be OK. If you want you can bolt on a Fram fuel filter down on the frame. If, when you drain the fuel it's dirty I would consider a new tank as most cleaning and refreshing tanks can be less than perfect and can get a bit expensive using the coatings folks seem to use. Re. LMC "reproduction" tank. I didn't use the cab mounted tank. Instead I bought an aluminum tank and installed it in the rear between the rails. For that reason I am not a good subject for information on cab mount tanks.
