i just finished doing something kinda like this. ive got a k4 and i didnt like the "latched" gas cap. i wanted more of a classic look, and while id really like a side fill, i settled on keeping the stock fill location but lengthing the spout a bit.
what i did was dump the fuel out and let it sit for the day. then i stuck an air compressor in the spout and blew the tank out for about 5 minutes. next i threw a lit peice of paper in the tank and itll burn for a bit while it uses all the oxygen in the tank.
after all that, the stock latch and assocaited brackets were cut from the tank. then i sanded where i was welding and it just happens that the harley bung i was using was the same size as the stock spout..i butted one to the other and TIG'ed around it. the metal is thinner than i thought..ive got a high frequency inverter tig and i ended up using a very thin tunsten and a max amperage of about 30amps. i was still burning little holes so i turned the frequency down and "pulse" welded. the main thing here is that butt welding sheetmetal is pretty damn hard unless you have a sheet of copper you can use for backing.
all in all, it came out just like i planned. now ive got a 2" tall spout with a simple, round 53-76 harley spin on cap powdercoated black! its hard to tell much, but heres a quick picture after i finished welding.
PS, if youve got the welding skill to do it; id cut the stock latch out, use a pnumatic flanging tool and put a flange around the hole, throw a peice of sheetmetal on and weld away. that would prevent any buttwelding and definently cut back on any chance of burn through.
