fuel tank

[whiting68]

I own a 1968 falcon xt gt which i have totally restored over a 4 year period i replaced every nut bolt washer & screw on this car .It has been lic for 7 months and has done 2,700 miles . Last week i took it to the perth motorplex to see how it goes down the quarter mile it ran a best time of 14.4 which i was very happy with for a mildly worked 302 windsor. When i was putting the spare wheel back in which sits in the fuel tank resease the tank had about 8 upward creases which looked like the tank had imploded or expanded , has anyone had the same and what has caused this