The linked page says 79-86, but they fit on any 79-93 hardtop. Vogtland doesn't list a spring for the 87-93 hardtops on their website.
This is what Vogtland part # fits what Mustang..............
Part # - Description - F/R Drop
953066 - 79-93 Mustang V-8 Exc. Convertible - 1.2"/1.2"
953067 - 83-93 Mustang V-8 Convertible - 1.2"/1.2"
953079 - 94-04 Mustang SN-95 V-6 Excl. Convertible -1.4"/1.4"
953068 - 94-04 Mustang SN-95 V-8 Inc. Cobra, Excl. Conv. - 1.6"/1.6"
953069 - 94-04 Mustang SN-95 V-8 Convertible Inc. Cobra 94-98 - 1.6"/1.6"