OK, I did some research. The recall included: tap a hole into the filler neck, attach a hard line in the neck and route to the edge for the trunk safely away from the exhaust, (wheel well?) Finish by installing a non-vented fuel cap. Reason was to prevent a fire from sloshing fuel out the vented gas cap, which was directly over the exhaust tips. Probably an over generalization.
GT500 only?
The fuel cap in the lower right hand corner is the one featured in Marcus's article. It did come from a 1970 California emissions Mercury Cougar. West Coast Classic Cougar shared parts for comparison, for research in his article.
Top left cap is a OEM factory 1969 Mercury Cougar cap, also the same as 1969 GT350?
Top right cap "ANTI SURGE VENTED". Stant stamped S, has a one-way check valve built into the cap. Auto parts store replacement cap or later Ford service cap for part number C7AZ-9030-A?
Ford MPC indicates 69/70 Shelby GT350/GT500 without ev/em, Vented-anti-surge-2.47" diameter, part number C7AZ-9030-A.
Bottom left cap "PRESSURE VACUUM". Stant and Autolite marked, also has one way check valve. Different service replacement?
Bottom right cap "NOT VENTED" Stant marked, does not have one-way check valve. 2.25" diameter.