That tank size just seemed wildly unreasonable to me, both in practical terms and in physical packaging: a 50 mile range is unacceptable, and the tank which I've seen looks larger. I've been on a road trip of 500 miles each way with a friend who was driving a C3, and he didn't have to stop ten times for fuel in each direction. So I did a bit of searching, and while one source lists a 4.0 gallon fuel tank (and 5.0 gallons if you get the big block engine), automobile-catalog.com lists "68 liter / 18 U.S. gal / 14.9 imp. gal", which seems a lot more likely... and is confirmed by many other sources.
The fuel tank size isn't really important, since it won't be used anyway. I think what is important is that "better than a 50 mile range" is your goal, and that you will be happy with that even if you realize that the stock Corvette's range is over 200 miles (or a bit less if actually driven like a Corvette).
By the way, the tank got bigger (24 gallons) later, by switching to a space-saver spare tire which allowed the tank to be deeper... and that suggests that a rear battery pack should use the full height originally used by the spare and the tank on top of it.
