1966 dart front brakes?

Ok, so! i have a stock 66 2 door dart. It has a 225 slant six, all original accept the hood and right front fender. I ordered front brake drums, and shoes. Turns out the drums are 3 1/2" wide. My factory are smaller, and the question is; what has this size of drum and is it an easy swap? Do I just need the backing plate? It is only about a half inch wider which would be an improvement in my book!! Any feedback on this would be appreciated!