Thanks for the replies on plastic tanks. I will follow up on this. The purpose of the fuel cell is for us to get the car running economically (maybe the stupidest word in the world when applied to GT40s, isn't it) and let me research the tank question.
I have some prejudices against metal tanks, which stem from what I have seen happen in boats. Now, to be fair, the situation in boats, especially here in the Chesapeake area where the water is brackish, is not the same as you would find in a car. But basically the problems are thus:
-aluminum tanks which are mounted in a steel monocoque will be subject to galvanic corrosion if the two metals come in contact. Other than removing the tanks and checking them, there is no way to continually verify that they are still serviceable. The process of removing them for inspection- sliding them in and out of the sponsons- will accelerate the process of removing whatever coatings are preventing the aluminum and steel from contacting one another.
-stainless tanks are subject to crevice corrosion in areas where the local oxygen tension is not high enough to maintain the oxide coating on the stainless steel. In marine metallurgy, stainless parts in underwater situations where the water is stagnant will develop crevice corrosion. Since water accumulates in motor fuels, and falls to the bottom of the tank, and there is no oxygen present, the potential for crevice corrosion and tank deterioration exists. However, removal of the tank for inspection is far easier, since there is no problem when the tank is slid in and out of the steel sponson surrounding it.
GT40s by their nature are left undriven for long periods of time; they are not daily drivers. Therefore, water and solids will accumulate in fuel tanks and possibly lead to tank damage over time.
In marine fuel tank practice, at least until recently, the gold standard was molded fiberglass tanks with a sturdy layup and made with a fuel-resistant resin. One manufacturer with which I am familiar (Hatteras Yachts) had a perfect record of tank integrity for decades- until ethanol was included in marine fuels. This mixture turned out to be lethal for many old fiberglass tanks, which deteriorated, leaked, and liberated various compounds that are toxic to carburetors, fuel lines, engines, etc. Bertram Yachts were similarly affected. Boats which had diesel engines have not been affected so far, but we don't know what they are planning to use to extend diesel fuels. We hope not ethanol. Ethanol is fine for drinking, but not so good for feeding to old engines, at least not through older fuel systems.
The best solution, at least in my opinion, is rotomolded plastic tanks which are really impervious to gasoline and ethanol and can be made in any shape. For those of us who are buying complete cars (also known as "people who are a lot more intelligent than people like me who spend eight or ten years recreating a 1965 automobile from scratch"), you are going to get the tanks that the builder puts in the cars. If I had aluminum tanks in a steel monocoque I'd change them out. However if I had stainless tanks in a steel monocoque, or aluminum tanks in an aluminum monocoque, I'd periodically drain them, I'd use a water absorber in the fuel, and I'd empty the tanks if I knew the car was going to be stored for more than a few weeks- like in the winter. And after a few years I'd inspect the tanks, inside and out. With modern fiberoptics this wouldn't really be that difficult.
Sorry for the long post. I have seen so many instances of metal fuel tanks leaking in boats (sometimes with very serious consequences including fatalities) that this is one of my pet worries.