What kind of finish should be on the distributor hold down on a 1970 440. Thanks.

Kinda hard to see, but here's a 17,000-mile original (except valve covers, since returned to stock) '69 RR clamp.
A '70 Satellite I had was the same, like a clear zinc over clean bare steel, so a dull medium silver...

Just so people don't get confused, my '69 340 distributor hold down is copper plated steel with a blackened finish. Birchwood-Casey, maker of gun bluing, sells a brass blackener that also blackens copper. It's easy to plate copper with some battery current and a copper penny or Romex.

One could guess the copper plate was to make sure there's a good ground between the intake manifold and the distributor. And in addition, for good measure, the intake manifold to head bolts were zinc plated with yellow chromate, rather than phosphated, likely to make a better path to the negative battery cable attached to one of the heads. And maybe the battery negative cable was bolted to the head rather than some place else specifically to make as direct an electrical connection to the distributor as possible.

In the photo, most of the original blackening is gone due to too aggressive cleaning and scrubbing. Oops.

clear zinc or silver cad

I painted mine a matte nickel color

I have come across copper ones also always small block.

Yeah, My 73 340 had a copper one under the gunk.