The lower ball joint from Magnumforce is just a regular disk brake lower ball joint. Nothing fancy or special about it, it wasn't designed to improve bump steer or anything for use with the drop spindles. The drop spindles were patterned after 73+ disk spindles, and that's the stock lower ball joint for those.
You'll have to excuse me, I realized that the early 9" drum spindles had a different bolt pattern for the lower ball joint, but I didn't realize the steering arms were also a different length. I don't have a 64-66 car, and my GT's are 10" drum cars which use the later style lower ball joint. So, I forgot that the lower ball joints change on the early 9" drum spindle cars.