My original filler was still in decent shape when I took the vinyl top off, but sooner or later it will let go. I'd rather it not be when I put a new top back on so I chose to replace it with fresh material.

If the factory stuff lasted 38 years then this should last just as long. It will be kept inside most of the time and rarely see water so it should be ok. I'm going to have the roof painted with a good thick coat of base-clear urethane paint so that should lock out any moisture. It has to be a whole lot better than the light dusting of enamel the factory gave it.

Filler lifting is caused by the surface not being cleaned properly most likely.