In 1970, the Pontiac GTO did not have a a coolant recovery tank from the factory. These were not a production item until about 1973. This does not mean you can not install a universal tank yourself. Ames Performance Engineering (https://secure.amesperf.com/) is a recognized leader in Pontiac restoration parts offers several tanks I would recommend.
PT# P134JS is a white universal style tank that installs easily on a flat surface such as the radiator core support, you will need to purchase some hardware to mount the tank but cost is $12. They also offer a stainless steel tank for $84, that mounts along side the radiator. It is a high quality kit that is complete and ready to install and looks really sharp under the hood. The PT# P133A. I would give Ames call for more information at 800-421-2637. Let them know your friends at Hagerty referred you!