I've been doing some searching and I came across this company that makes aluminum tanks, Boyd Welding in Ocala, FL. It appears these guys have done many types and many tanks. Prices look fairly reasonable also. They do make a few Factory 5 tanks already. They also sell tank parts. Check out their gallery.
Boyd Welding LLC
Factory 5
Aluminum Gas and Diesel Replacement Fuel Tanks
Gallery
Custom Aluminum Diesel and Gasoline Fuel Tank Pictures
Ralphy