OK custom tanks aside...The thing that I have seen is take the fuel tank off of a semi along with the frame cradles and fuel tank straps and mount them in the bed of your pickup and use a stock diverter valve to switch over to the bed tank.
I have seen one that the guy actually stopped using his factory tanks and ran a 120 gallon semi tank in the bed of his pickup and never complained about anything other than how much it cost to fill his tank.
Also it has been awhile so deisel weighs in at 7 lbs per gallon, If I am wrong please excuse me, considering that the 120 tank ,and using 110 gallons each fillup, would be mounted at the front of the bed center, these are rough figures ok, Fuel would be 110 glns=770 lbs so split that in half for a standard cab gets 385lbs per axle apprx. for a crew cab since it is closer to the rear 513.3 lbs rear axle 256.6 lbs front axle apprx.
Lastly let me say this is your truck and your decision and you should consider the choices and opinions of others however you have to live with it so it is your decision to make.
Good luck