If I change my cluster with a used one from junk.

Mileage is recorded on the cluster.

You'll have the mileage that was on the donor van. There are places that can reprogram the mileage to what is currently on yours but I don't know if they require any kind of documentation or if they would accept sending the old cluster along. Dealers order clusters and provide the mileage, then it gets delivered already programmed.

I used to deal a lot with Downtown Radio of Denver for Chrysler radio parts. I visited the place in '99. They do cluster repair and reprogramming. You can also look at United Radio of New York.