Are the Sierra brand XR u joints 18-6407 as good as factory Merc. Or is there any other non Merc joints that are as good? Do I even need to replace with same later (GKN) style joint? Thanks for any input.

I have used the sierra u-joints for years with out any failures, on stock and modified engines. I'm not sure who makes them, but they seem to be just as good and the stock ones.