Yes, Rick,
I've got the same problem as you have.
I have seen the bracket in a restored 280 SL last weekend, the plate for the extra fuse box was yellow cad plated and the fuse box was attached to it with two M4 or M5 bolts and nuts.
However, the original scenario was with (hollow brass) rivets.
I have two used brackets and both are not cad plated. One is factory painted black, the other one rusty (I don't have it in front of my memories-eyes now but I believe to remember that it wasn't cad/zink plated either.
So, it's all up to you. If the bracket is heavily rusted (and also underneath the fuse box), you may want to drill out the rivets and do whatever you prefer with the bracket: plating or painting.
If the rivets are still good and tight, just respray the remainder of the bracket .
I most likely go with the later; I prefer the original look.
My humble 2 cents...
Achim