Why make a new one or buy some high dollar tank? I found the 67-69 camaro tank (18 gal) fits perfect between the rear frame rails of my 54. Other options include 69 or 70 Mustang tanks and 74-81 camaro/firebird tanks. Unless you are building a show truck I would put the $300 you can save towards another more visible aspect of your project. Just my $0.02.
There are several sites that show donor tank installations/uses. A yahoo or google search should find them.
If you want to make one yourself go for it. I saw one site where the guy made his 20 gal tank out of 14ga steel but I would think that 16ga would be sufficient also.
