Here is what I was talking about for a juice can gas tank. Silverbear made a lot of them based on one made by a member called, Rockenstein. An apple juice can is probably the easiest to clean.
Punch two small holes in one end to drain it out. Use a 3/4" hole saw in a drill to make the hole for the filler. Solder in a 3/4" copper female threaded pipe fitting using silver solder. Silver solder is stronger and works better.
Thread a 3/4" brass hose connector into the copper fitting. These fittings are threaded on both ends and have a ring in the middle or sometimes a multi sided ring so you can tighten it with a wrench. The cap is threaded brass with a knurled surface so you can turn it on to the brass connector.
These can be found in Lowe's or Home Depot's plumbing section as well as any good hardware store.
Buy a pet cock from the internet or a lawn mower/automotive supply store and get a matching fitting to solder into the can from any of the above suppliers. Brass or copper are the easiest to solder in.
Hope this helps and if you go to the google custom search at the top of the page and put in "Juice can gas tank" you will find a lot of tanks that people have made.
Here is one of the ones that Silverbear made.
Steve.
http://motorbicycling.com/attachment.php?attachmentid=22946&d=1270900139