1965 F100. The wipers are slamming hard on the cowl. I've tried reindexing the arms on the wiper shaft but that doesn't help. Is there an adjustment on the motor to get them to reverse direction before hitting the cowl?

 

First you need to determine if there is excessive slop in the wiper linkage.
If so, it has to be replaced or do something to reduce the wear on the bushings.
If not, remove both wiper arms from the drivers. Then operate the wiper motor so it goes to the correct position when shut off , then install the wiper arms on the divers so the wiper blades are all the way down to the bottom of the windshield at the normal off position. If the arms are the correct ones and there is no excessive slop in the linkage the wipers should be in the correct operating range.