The 67 convertible has a different method of securing the leaf springs to the axle housing. It has additional upper and lower spring insulators with an insulator retainer bracket that mounts to the perch on the axle housing. The insulators are rubber and the bracket retains them in position.
What this all boils down to is that you are going to have to replace and reposition the perches to the coupe's attachment method. The insulators and related bracket along with the mounting plates will need to be eliminated and the coupe's required parts used. The spring index pins are also larger on the convertible, so the indexing hole in the perches are larger. This will affect the axle housing's alignment if the current perches are used.
A comparison of the perch positioning and the index holes on the old axle housing should show the difference. A pictorial of the convertible spring mounting method is in the 67 Mustang Factory Chassis Assembly Manual.
The reason for the difference is probably due to the additional weight and chassis noise, vibration and harshness issues that are unique to convertibles. | 
