I'm not sure i'm reading your question right but remember that a mustang is a unibody car ... that means it has no seperate frame. You can't just remove all the front end sheetmetal and replace it with fiberglass because that sheet metal is the structure of the car. Without that sheet metal, (raditor support, aprons, unirails, wheelhouse assemblies, side rails) there is no strength. All that sheetmetal is what takes the place of the frame. Also when you get hit that hard they have to put the car on a rack and pull the chassis back to factory specs ... not a cheap process ! and never try to make a vert out of a hardtop without strengthening the rocker panel area ... verts have extra metal in the rocker panels to make up for not having a roof, which is part of the structure that gives strength to the car. You can usually change fenders, hood, deck lid and bumpers, but other than those, most everything else is welded in place and should only be attempted if you really know what you're doing. So to sum up this long winded responce (whew) What you're proposing doesn't sound at all cost effective.
zzzzzzz are you still awake ? lol