Get lucky and score one from a 454SS; bear in mind they're kind of uncommon.
Or, use a 6 lug 14 bolt from a C2500LD and either redrill the axles (or buy custom ones) and redrill the drums.
Or, say F it and convert your front rotors to 6 lug as well so you're 6 lug all around. Bear in mind not all modern 6 lug wheels fit the large old style hubs due to the center bore size and depth.
Richard