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Buddy's suggestion to get an idea of what you're using is a good one. I plan on using a dual set up over the wheel wells so with the Methanol additives in the winter, I'll see the traction benefits w/o having to add sand bags which have to be moved to load anything.
Thanks for the link Guybb3, I'll explore that one in depth later on when I get the current projects finished...if ever. :rof:
Here's a couple to you visualize what you may want to eventually use.
9 Gallon tank mounted at the tailgate. Local driving below 1000' alt.
25 gallon water tank mounted under a 30 Gallon Propane tank next to the cab.

Tool box sits behind the figure "8" set up with the Zex Nitrous system cleaverly hidden within.
Truck w/ Gm-8 gets 23 MPG on WMI alone cruising at 62mph which is where he likes to drive. On our road trip last AUG we maintained 75-85mph. So, that 23mpg he
normally got...not so much. We'd consume about 1:1 fuel to H2O plus the propane. Something to consider if you're towing cause we were loaded out at 8500lbs plus and not towing a thing but my big ars and traveling at altitudes above 4000 and up to 8000 for most of the 5000 mile trip.
LSR w/60 gallon center mounted in the bed for stability at speeds in excess of 155mph. :hihi:

Go big or go home!
Actually, the smaller tank in the lower left corner is the one the motor uses for the actual runs. But, the 2500 he uses for hauling has a 60 gallon tank mounted in the same position to sustain long x-country runs.