Here's a helpful link I've referred to in the past;
http://www.oldride.com/library/1964_pontiac_gto.html
And the area of your concern states;
Dimensions & Capacities: Wheelbase is 115 inches. Overall length is 203 inches. Tread is 58 inches, front and rear. Overall width is 73.3 inches. Height is 53.5 inches for the Sports Coupe and Hardtop, 53.6 inches for the Convertible. Total trunk capacity is 32.1 cubic feet. The gas tank holds 21.5 gallons. Shipping weights: Coupe-3272; Hardtop-3292; Convertible-3422.