The 69-70 fastback has a rubber gasket and the factory used some type of sealer in addition to the gasket. Your problem with the moldings may be due to the rubber gasket being too thick or the moldings could be made wrong if they are reproduction ones. Even if all your parts are original, the back glass molding are a challenge. I would suggest you contact a reputable glass company that either deals in older cars or check with some other classic car people to see who they use for glass work. If your using reproduction moldings I would try at least one original molding to see how that fits and go from there.