Hi David, I'm glad to assist you. As per checking, this fuel tank is a guaranteed fit for the 2014 Ram 2500 Big Horn 6 Cyl 6.7L diesel engine only. This will not fit the 5.7 and 6.4L gasoline engine. We do not carry the fuel tank for the 5.7L and 6.4L gasoline engine. We actually have in stock a related part you might need, this is the fuel pump assembly, with part number DELFG1432, by Delphi brand. Please check locally for availability of the correct fuel tank I hope this helps. Happy shopping. Please click the link below for the fuel pump if you also want to order that fuel pump. https://www.carparts.com/details/Ram/2500/Delphi/Fuel_Pump/2014/Big_Horn/8_Cyl_5-dot-7L/DELFG1432.html?can=CPJE1043

Hi Mike, The Mega Cab rides on the same wheelbase as the Crew Cab and yes, this will fit a crew cab (51 gallons). For accuracy of fitment - On the Homepage, please enter (complete) your vehicle’s year, make, model, sub-model, and engine size. Type the part's name in the search box. Once your application is selected, you’ll see a “Guaranteed to Fit” badge if the part will fit, and we recommend that you also double-check Your Vehicle Fit Details as well as the Vehicle Fitment Information table on the product page.