Will a 67 Designs device holder ball fit in a Ram Mounts® arm?

No, but there is a better solution.

The (CF) Carbon Fiber Series system is standardized on the 20mm metal ball and not the 1" (25.4mm) metal/rubber ball Ram uses.

However, you can attach any Ram Device holder using our 67 Designs' Dam Adapter™ at the end of the carbon fiber series arms. 

Dam Adapter™ (https://www.67d.com/products/dam-adapter)