When you build the tank make it small in plan view and tall in side view, make a sump at the bottom for the fuel pump inlet and if you are using EFI make sure that the return from the fuel pressure regulator dumps the return fuel below the fuel level and as far away from the pump inlet as possible and maybe even put a small baffle between the return area and the pump inlet sump. Use only stainless lines, a little bit more of a pain in the a$$ but much more reliable especially at the salt. When you build the tank you don't really need a break you can bend up the sides with a pipe or angle "iron" clamped to a good table. Heed Freud's comment about the corners not being a sharp edge, very difficult to weld and they like to fail. You need to buy (or make one yourself) a tee dolly which you use to radius the edges before you weld them, then the weld becomes a butt weld much easier to weld and much stronger. That is probably how Jason in Freud's pic put the radius on the tank he is welding ( he may also have an edge radius rollers for his bead roller). If you happen to make the tank from aluminum, back weld as many of the joints from the inside of the tank as you can (I am assuming that you will TIG weld the tank) as it will reduce the number of leaks that you will have. If you plan the tank out right you can do all of the side welds and the bottom welds from both sides (i.e. back weld) then weld the top on, again radiusing the corners. I have built a number of aluminum gas tanks and have never had one that didn't leak (maybe that is a statement on my TIG welding ability!) so when you are done fill the tank with water and mark the leaks then weld them closed and check again. I have an aluminum gas tank in my little lakes roadster and have put probably 15,000 miles on it without problem but I check it alot.
Rex