I do not have a 3878972 3-27/32" dia. yoke that you need, but I have a 3879208 4-5/16" dia. yoke for my 67 and it has no plating of any kind, just like the drive shaft. Forged casting and steel stamping. Does have a green paint dab on one of the ears.
I would think, with the cap in place very little paint would be ion the end of the 3rd member under the slinger. Reason, probably re-used the caps.
When I paint the complete housing, I will tape off the yoke @ the outer slinger, stand back about 2 to 3 feet and let her rip.
Bob