I don't think the vin number security issues with the Mopar stuff started until the mid to later 2005-2010 stuff.
Its very likely the instrument cluster you bought will plug into the wire connectors in your harness.
The age of the 56 would void any mileage requirements that exist, if its titled as a 56. Beyond that, your rebuilding most of the drivetrain on the truck, I wouldn't be too concerned about mileage.

Way back in the last century when I worked at a Chrysler dealership, we changed a speedometer in a 2-3 year old still under warranty car and all the was required then was a sticker stating the original mileage, and the current mileage on the odometer with the date the change was made.