Hi Maddie, glad to see your making a start on the mack's. The Mack would sit straight on the king hauler chassis it all depends on what look you are after, my mack was built on 23 inch rails and tri drive with a sratch built sleeper pod thats about 1 1/4 inches longer than the king. My guy in the uk is Kustom Ed on here.....Here's a photo of what it looks like on a king chassis for you :-)
Hope you have as much fun as i did biulding mine