only odd drum I remember is the keyed axle drums or the 12"x3" drums on heavyduty dodge trucks. I had a set of those on a early 80s truck with 9 1/4 rear axle.

need to know what axle is under there? what drum size is on it now?

if none for a 66 then swap on some later backing plates and drums of a 8 1/4 8 3/4 9 1/4