@FE468Stroker is dead on.
There were two basic radiators available in '67 and '68. The one you want is referred to as the "big block" radiator, and should be a 3 core 24" wide copper/brass unit. When combined with the standard big block 7 blade fan and thermal clutch, along with a proper shroud, it'll keep your engine nice and cool with no fuss at all.
The smaller 18" standard radiator would not be adequate, especially on hot days in traffic.
I would avoid going with an aftermarket solution here if you can; most of them are too thick and obstruct airflow through the cores, or just oversaturate the air with heat so the extra thickness doesn't help. Aluminum radiators tend to corrode away unless you take coolant changes seriously, and use good quality antifreeze; aluminum will try to sacrifice itself to 'repair' rust in your cooling system. When you combine an old engine with a new radiator, this often leads to issues over time.
As an aside, you can readily find all of it at West Coast Classic Cougar. Most of the Mustang and Cougar drivetrain parts are interchangeable, and this radiator setup is no exception. They list it for both cars.