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.My basic goal is to leave the car better than it would have been stock, which shouldn't be hard. The stock motor made all of 190 HP, and with a measly 4 gallon tank and 12.1 miles to the gallon, it had a paltry fuel range of ~50 miles.
So, Around 200 HP equivalent, 250+ ft/lbs of torque, and better than a 50 mile range, and we're golden.?
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.