I don't believe anyone make a "kit" for the big trucks, you would need to pull it together on your own. Your '56 6500 MC has a 1-1/4" bore, so that's the bore size you need to look for.
If all of the parts are new you should have many years of good service from the original system.
Make sure all of the lines (steel and rubber) are new as they fail from the inside out, replace/rebuild the MC and wheel cylinders and go through the shoes, adjusters and springs.
Don't forget to make sure the parking brake is in good working order and adjusted as well.
Mike B
