How to go about replacing roof on '68 dart hard top

You could find every single spotweld on the roof skin and drill them out to remove the skin but I think that replacing the entire roof at the pilars would be faster IF you get it right. You need to take a million measurements and test fit every thing (doors, glass, trim, etc.) before proceeding with your welds.

As far as the procedure. I don't think that I can explain it. It's something you need to see and do for yourself. You might want to ask a body shop how much it would cost for them to do it. It's something you really don't get a second chance at getting it right.