Regarding the rattle traps of Fox Body Mustangs and that same era Camaro, yeah, they were bad. Even when fairly new and especially if you had a convertible or T-top.
But that was many generations ago, back before Detroit put so much engineering effort into them like they do today. Today's Mustang and Camaro and even Challenger, offer up a significant improvement over that era, let alone 60's muscle. A very, very significant improvement. And that's just in base trim models. Performance models take it that much further yet, through additional stiffening, lightweighting, tuning, etc.
Blu, we will forever disagree on this matter, but what I know as a systems engineer, is that to best tune a system to perfection (or close to it), you have to consider the whole system as you do it. No drop in system can do that.
And 200,000 miles from now those brand new cars will be rattle traps too, and they will need parts replaced and upgraded. A systems engineer should know that. Heck all the of the 2011-2014 Mustang LCA's are still under a TSB for constant squeaking and bushing failure. That's pretty much the modern era yes? New designs don't always mean better designs.
The biggest hurdle to overcome isn't usually the design of the classics, it's the age and wear and tear. But that can be dealt with if you're willing to do the work. New design methods are more efficient, but they don't always yield a better product. Innovation in aerospace dropped 50% with the introduction of CAD systems in the eighties. Even engineers have a hard time understanding that just because their computer program can't do it doesn't necessarily mean it can't be done. It just means the software wasn't programmed to do it, because the guy programming it didn't have the required imagination and/or knowledge. We sent men to the moon with less computing power than a modern graphing calculator, let alone a smart phone. Those engineers knew how to DO things, not just optimize a model on a computer that may not be programmed with the proper assumptions. The fact of the matter is that all of the suspension systems currently in use, even in brand new cars, were invented at least decades ago. Sure, the individual designs are newer, but there's nothing new about IRS, or coilovers, or anything else out there today. Jag's got IRS in 1961. Heck Firestone marketed a practical air ride suspension in 1933. None of this is new.
You won't hear much argument from me on the drop in systems, that's why for the most part I don't suggest them. Slapping coilovers on a chassis designed for torsion bars causes as many problems as it solves, and those issues have to be addressed. Upgrading the stock design works within the original system, and optimizes it to work with modern tires and compounds. Again, a systems engineer should understand that. Installing tires with better compounds completely changes how the entire suspension system behaves. You must make more changes to it in order to keep it functioning as intended, and even BFG T/A's are an upgrade from the tires the suspension was intended to work with. Meaning, unless you're running bias ply's you need to upgrade your suspension to get it to work properly.
You can disagree all you want, but the car sitting in my driveway right now disproves most of your claims outright. And as a former aerospace engineer, your systems engineering title doesn't impress me unless you can back it up with knowledge and ability. I knew plenty of engineers that couldn't build something with their hands to save their lives. Sure they were great on the computer, but hand them a chunk of metal and send them to the machine shop to actually make what they just designed and you were in deep crap. The computer model isn't out on the road, the chunk of metal is.