and now my first wheels question

A fairly common problem that used to be experienced with A-body Mopars was custom wheel clearance on brake drums, especially the slotted chrome wheels. Manufacturers failed to consider the small drum diameter so the back of the wheel would actually contact the casting on the outer edge of the drum or, as mentioned by Waggin, the drum weight which distorted the drum and caused brake pulsation. It was common to make a spacer of aluminum with a diameteter that fit against the smooth surface of the drum, and fit inside of the outer cast ridge, just thick enough to move the wheel away from the cast edge of the drum. I would guess that is the plates referred to above.