Heck,a warning for over 120 days? These old tanks need some repairs where this information is important.
I can't say that I know, but your best source for material is from another tank of the same or similar vintage. My experience with this is through a mentor who was making a cutout to fit a Mikini slide carburetor on an f-1 ducati. His choice for steel was from a volkswagon beetle (perhaps a fender) to make a concave area in the tank to fit the downdraft conversion. Others who did the sameconversion copied the tank in Aluminum with the similar modification. He claimed knowledge that it was steel of similar alloy and thickness, and the job is done. It worked good.
I would suggest getting some forceps ( compass, micrometer, etc) and perhap even another tank to use for your materials.
