I was thinking it was only about $15 for the 3M rope/ribbon too.

I've been told to stay away from the urethane. Urethane is for later model cars (80's and up) that use the windshield to strengthen the structure. It makes a solid, no-flex bond. Our cars weren't designed that way, and the windshields aren't made to withstand a lot of twisting and stretching. If you use urethane, the windshield will eventually crack or shatter as the body flexes.