way to buff out barracuda trunk plate?

Are you talking about this?

http://www.rrclassiccars.com/p1969barracuda11.jpg

I don't believe it should have a shine. The large center area would be a satin silver.

Get a can of Krylon #1403 Dull Aluminum spray paint (The match will me more than close enough to the factory look for any street car).

Tape off the chrome edges.