5200 would be fine, as you should never have to remove them again. 4200 UV would also be fine, and would resist crumbling on the edges longer. I would not paint
them, but instead, polish them out like chrome! Also, be sure to use 316 SS bolts to match the chainplates, or they become the "anode". (Fastenall) IF you can't fit backing plates
, or they wouldn't lay flat with uniform contact, I'd use a stack of first a 1.5" and then a 1" fender
washer under each nut. Torque down the bolts like it was the head
on an engine
... "opposing corners". Once snug, you can leave it alone, but IF you prefer to have a "loose snug" and put another 1/4 turn on them a week later, be SURE that the bolts do NOT turn at all, or you break the seal at their heads.
Good luck with it!