I forgot to mention I like your buck.
Like AllyBill mentioned use 1100 X.050 to make your first tank outer portion. Learn what steps are necessary to get the tank you are after (even the welding and finishing). By doing that you will find out what tools are required and what tools you need to add to arsenal.
Once you are satisfied with your skills, then make your final piece using 14ag. for the inside and no less than 16ga. for the outer. If it were a Harley I'd say 14ga. all the way.
Jere