With vintage muscle cars being 35-plus years old, it won't be uncommon to find the factory rubber body bushings dried out and in need of replacement. As the bushings degrade, the body will slowly sink and the panel gaps will start to change. You could replace the worn components with rubber during your restoration, but those will eventually dry out as well. Luckily there is another option that will outlast rubber and also stiffen the chassis for better handling capabilities.
We are talking about a polygraphite body mount kit from Performance Suspension Technology (PST). These bushings are stiffer and less likely to be eaten away by things like Father Time, oil, and road salt. If stiffer is better, then why not just solid mount the body to the frame? Because the bushings not only cushion the body, but also isolate road noise and vibration. A solid mount will transfer all that nastiness right into your interior. Plus, there has to be some sort of give or things could start to crack or even break. The PST bushings have a good balance of firmness and give, and handling is improved without things getting too loud or damaged.
With these components being under the car and exposed to a pretty harsh environment, the hardware could also be on its way out. This was the case for the '66 Chevelle we have been tinkering with. Not only were most of the bolts flat out missing, the few that we did have were rusty.
We picked up a new hardware kit from Original Parts Group (OPG) instead of trying to guess what we needed and hoping the hardware store had any. The kit came with the proper bolts and washers needed for the job, but we ended up using the washers from the PST kit. All the parts set us back a little over a 120 bucks and the job took an afternoon to complete.