With your being a 51, it more than likely has the "Huck" type brakes on it. Starting in 53 GM started using "Bendix" style brakes. It's possible yours got converted to the newer style, but since you've owned it for 40 years, I doubt it.
Either way, the 3600 brakes and drums are hard to come by. I just did a search on Jim Carter's and it seems they have the parts you need.
