Ok, first a little reality check here...You've got a 71 coupe that you want to make in to a convertible. First thing your goping to need is a new set of rockers, both inner and outers, next, torque boxes for the convertible, new quarter panels, new interior rear panels, and a host of very small, not yet reproduced sub brackets and reinforcements. To say this is a major undertaking is an understatement. You really want an idea of what it takes, contact Fred Galzier at Glaziers Mustang Barn in Souderton, PA...Way back in 1982, he did a coupe to convertible conversion on a one owner 68 coupe...The cost back then was 20K+ (plus the car itself).....I would think twice about attempting this on a unibody car such as the 71 Mustang......
Just my two cents worth on this particular subject...