Most Dearborn made 1968 Cougars had a nifty piece of stainless steel trim that snapped into the drip channel on vinyl top equipped cars. It looks a lot better than the black goop and is 100 times easier to install.
You can peel it out of a car at a local wrecking yard or get some from Don Rush, John Baumann, Lin Thurston, Robin Jacques or one of the other used parts dealers.
If you want to reproduce the rubber goop look there are several brands of sealant that work. Go to any aviation parts supply and ask for sealant that meets Mil Spec 8802 B 1/2. It is a two part rubber sealer that dries black in color and stays rubbery. Expect to pay $15 - $25 for a kit. Be sure to mask off the stainless drip rail outer trim and the roof. Wear rubber gloves or youwill wear the sealant for a long time. It dries in 1/2 hour (notice the 1/2 in the part number...).