Build a wood form out of stacked up plywood in the shape you want, It needs to be little smaller to account for both the thickness of the tank metal and "operator error". Cut metal panels for each side, top and bottom. Allow about 1/2 to 3/4 inch extra all the way around on both side panels. Screw the side panels on to the wood block, now comes the sheet metal work, each side and end has about 3/4" too much, you will have to bend this over the wood form. Good flat head hammers are available, but a claw or ball peen hammer will work. You will need to notch in some area to keep the panel smooth, you are on your own for that.
After you have both side panels done, cut the top, bottom and end panels, also make your cuts for the filler cap and petcock. These should be as close to size as the top, bottom and end as POSSIBLE. Remove the 2 sides, drill a few holes and screw down these panels, over lapping as needed. Now replace the side panels on the wood form. This should give you a tank with overlapping joints on ALL of the connecting pieces.
Next use some self tapping sheet metal screws, about 3 per each side top and bottom, plus as needed on the ends. Remove all of the screws holding the panels on the wood form and the sheet metal screws. Next, weld in the filler post and the petcock connection. Use teh sheet metal screws to put the panels all back together. Now you have to weld all of the seems, try doing little parts at a time, an inch on all panels, then another inch. You do not want to get to much of a warp in the sheet metal. Once you have all of the seams welded, remove the sheet metal screws and weld them shut. You tank is almost finished, now get out your grinder and smooth out all of your welds.
At auto parts houses they sell gas tank sealer kits, I highly recommend using one to seal the inside of the tank.
Mind you, this is my way, it worked for me, but there are other ways to build gas tanks. But it has been 20 years since I made one and it is still going strong..
