Super cool advice, thanks!You have a 27 year old non-luxury car with performance mods and a loud stereo. You are fighting a crazy uphill battle, but I can appreciate what you want to do. I freaking hate buzzes and rattles too.
You seem concerned about weight and also making the best use of your budget. To make sure you're putting attention and materials in all the right places, here is something you can try. Download one of the free signal generator apps (sine wave generator) for your phone and make sure it allows you to gradually sweep through frequencies as if you were twisting a knob, not having to type in the frequency one at a time. Play that in your car stereo at a low/medium volume starting around a few hundred hertz and slowly lower the frequency until you hit 20hz. Then turn up the volume a tiny bit and do it again.
As you sweep the frequency every single rattle and buzz in your car will come to life one at a time. You can sweep the frequency up and down real slow to get it just right, then crawl around the car to find the problem. This is kind of like an acoustic spotlight. Fix the problem with the butyl rope or whatever product you like (Queef has some ideas too), then repeat at least a hundred more times.