Which tire was installed in production on a Z/28 or COPO 9737 was purely random. Both were stocked in assembly. The only requirement mentioned in the assembly manual was that all had to match. Goodyear WTGT was the predominant brand; Firestone SC 200 seems to be secondary. A friend bought one of the last '69 Z/28s built; had SC200s.
Having been around in those days I know that Goodyear was by far a more popular choice as a performance tire. Not at all unusual for a Z/28 buyer to request a wheel swap with another car in stock to get Goodyears.