The belt will not stay on if you take it up to 6500 rpm, I know that.
I would leave it alone. I used to get flack over what many told me was the wrong redline tach in a 67 Z I had but it had been there forever so I left it that way. Now I think there is a large cross section of others that also came that way. Makes for nice conversation also....
I'd change it based on Jerry's quote. 6500 rpm is nothing for a Z/28, that's right in the power band!