[funrunr]

I suggest removing the gasket and installing a new one from Carpenter or reputable company (NOT MAC'S). If your replacing both sides place tape on the outside center of each piece to later identify. Cut rubber with Stanley knife from the inside of window along the edge of the rubber gasket all the way around the window. with someone on the outside begin to push the window out and the helper outside will "catch" the glass. Now is the time to clean the metal edge the rubber secures to and straighten if necessary. Look closely where water was leaking.
Next, use hot soapy water and install the gasket onto the glass. (larger flatter rounded lip goes outside). Soak the pull cord or small rope in soapy water. Line the inside lip with the rope all the way around and overlap a little leaving a foot of extra rope hanging loosely. (ideally start and stop in the middle and not in a corner) Place the glass from the outside with bottom edge resting somewhat in place. Have the helper push on the glass firmly (they are only stopping it from falling or popping out, it does not take much force) from the outside while you begin to pull the rope from the inside. This will make the rubber begin to overlap the metal frame/lip. Once done, if the rubber is still wet you should be able to slightly move and adjust to make it look straight and neat. Allow to dry overnight before testing.
Baby shampoo and water is a good mix, I've heard others suggest baby oil but I never used it and don't know if it would dry overnight.

Below is the "get by for now" solution and I don't recommend.
They do sell a clear silicone for sealing windows, it is thin and will get into tight areas. make sure to tape off all edges of surrounding paint before applying. Insert tip of caulk tube behind rubber gasket and squeeze into seal around edges, swipe excess off with finger and wipe rubber clean with rag, then remove the tape for a nice clean look. I've done this to my daughters Ford Ranger and it works. Your car is a classic and deserves to be done correctly. It "could" be a nightmare to remove later in the future.