Neither conversion is as easy as anyone will make it out to be. Most of the time all of the pieces required to make it complete and look right are not purchased and changed. Then you have incomplete abominations that are desirable to no one and worth little.
Regarding the 'nose' conversion, you may think it's just a bumper cover but it's not. The physical bumper has to be changed. The same era hood is required. None of the rest of the bodyside moldings will match. Neither will the rear bumper cover. If you want the GT ground effects, holes will have to drilled in the body to rivet the retaining hardware to the body. When all of those things are changed the paint is all mismatched.
Regarding the 'quarter window' conversion, the roof rails have to be changed, there is no molding on the top edge of the doors to mate to the windows, and the interior quarter panel will have a large gap between it and the window, and the mounting studs will be exposed. If you want to add the door molding, holes will have to be drilled. Then the mirrors won't match. Manual mirrors are hard to find and a switch for electric mirrors is difficult to put in a suitable location. If you change the interior quarter panels, some new screw holes will have to be drilled and your existing cargo cover/privacy shade and carpet won't fit. And a new hole will have to be drilled for the front upper seat belt anchor, and the old hole plugged. If you change all of that, the rest of the interior color won't match.
It is far easier and less expensive to buy the car you desire and sell the car you don't.
1979-1993 Fox Capri and Mustang VIN Decoder: foxchassis.com
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