[2Utes]

The original stuff is about 3/4 inch wide and a little more than a 1/16 inch thick. It is also fairly sticky but very easily squished. It seems to provide a good seal for going on 4 years now for me. Some of the old stuff was still flexible when I took it off. Since the undercoating was still continuous, I think it was the original from 1982.
I am not sure what dum-dum caulking is, but if it stays pretty flexible and can be shaped into a thin wide ribbon, it could work pretty well. That said, the amount of work involved in re-doing the seal if it does not work is pretty painful.
Whatever you choose, be patient with the reinstall of the fenders and be really clean to avoid scratches.