do you have a book on the 68 mustang.. i had a 66 mustang since it was new and kept it about 10 yrs. and in the old book i HAD, it showed how the windshield along with all the other glasses who they were placed into the car.... i do know from hearing this along time ago.. lived across from a body shop when a kid.. the guys at the body shop were real good body and fender, and painting guys, but when it came to windshields they would break them so they just got a company to come out and do it for them.... if you have any metal deformaties in the area where the glass going to go it will break as soon as you put pressure on it....... if you already have the glass why not go to one of these places and ask what they would charge to put in the new windshield..... might be the best idea>>>>>