[dsraven]

take the tank to a rad shop.they will boil it out (probably needs it anyway) and solder in whatever fitting you supply wherever you want it. take the sending unit out first. no kaboom that way.
another way would be to clean the tank. fill it with water to the top, so no vapour, then solder or weld on the fitting of choice. after that is done and you are satisfied there will not be a leak, you can drill the tank through the fitting. then drain the tank, add some methyl hydrate to help absorb the water, drain again and let sit somewhere warm to dry out completely.
for a return line it may be nice to have a short tube inside to direct the return fuel away from the sending unit float, they aren't attached all that well. sort of like a dip tube in a hot water tank.