Roof cracks

The use of lead in those seams ended around 1970. Too many on the job injuries related to hot lead. Finishing body filler takes a lot more labor/time than the lead did so all the mfgrs' started pushing vinyl tops to lessen the labor/time required. The vinyl hides imperfections that paint would show.
Todays body filler is much better than what was available back then. If its properly applied it should last a lifetime.