Since I show my 66 in the MCA unrestored class I was under the impression repro parts are not acceptable thus the reason for wanting to find good used parts or NOS.
Buy new repro clamps and "age" them (just the opposite of "restore"). Dunk them in water and allow them to rust for a couple of months. You may have to repeat this process. Remember, a 45 year old original muffler clamp has been subjected to heat, cold, moisture and dry weather. A "good" used one is almost impossible to find.
Jim