I did this once. You just grind the top of the pins off. Drive them down with a punch. Remove the old bushings. Put new busings in. Then insert new hinge pins and cotter pins. I was stuck using some crappy tool and it took a long time. My guess is that a Dremel tool and cut-off wheel would work best for removing the top of the pin. Air hammer could work, too, but leaves more potential for a mishap.
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351W 89 Mustang GT Convertible