mike,..look into rolling your end seams instead of welding ,..if your putting a baffle in,.. there are a few ways you could attach it,,prior to forming your outer skin you could weld tabs that would hold the baffle in place and put a square hold where the tab could be folded thru the baffle to hold it in place and also limit expansion of the center section of the tank,..by doing this there is less chance of a leak because your flush welding the interior of the tank to a tab,,which will allow more flex on the baffle without stressing the baffle and weld joints,..ie from fuel sloshing back and forth putting side forces repeatedly,
get a good paste type sealer and put it on your end caps then roll a bead that forms the outer shell and end cap lip together,,this will allow the tank to twist but not leak and no welds to crack under stress,.. I'd use 3003 aluminum to make this,..if you have or know someone that has a French wheel it would make it allot easier to roll the seams,.. the skin I would overlap on the top in line with the filler port and do a folded seam with the same paste sealer in the joint the end caps and baffle would keep it from coming apart once installed..you could even solid rivet the over lap joint of the outer skin,.. and also solid rivet the tabs to hold the baffle,,with no welding at all
because of the area and weight of 13 gallons of fuel I'd use at least .020 thick aluminum or even .025