I live in rainy western Oregon and drive my truck in the rain pretty often. My wiper motor is original, I think, and works just fine. It slows down a bit under acceleration but it has enough power to really fling the wipers at high speed. I think you need a rebuild.
I'm planning to add a two stage fuel pump to get some vacuum on on-ramps but there aren't too many situations where it drops to zero on the 261 with an original carb. It could definately use a six pack.
Rain-x does help but it doesn't last very long. Teflon wiper blades help too.
Oh, and it doesn't rain all the time here, we haven't had any measurable precipitation in 6 weeks and are getting a little worried. Tomorrow looks promising though.
