Tony,
As far back as the earliest "stock car" races before the formation of NASCAR, most of the winning cars had been completely torn down, frames rewelded, springs rebuilt, and other parts replaced with "heavy duty" replacement parts. Things like axles, brakes, spindles, etc. were either "police/taxi" or "export" parts.
Engines were torn down and reciprocating assemblies balanced, all internal clearances optimized (blue-printed) and then reassembled.
NASCAR's first races were not for "moonshine cars" as so many folks would have the fans believe. They were for either modified or sportsman cars; the strictly stock class came only in NASCAR's second year. This is when all the hoopla about the whiskey runner's cars came into being and this is the very short period that the race cars might be driven off the street and onto the track with some chance of being competitive.
During the 1950's it became apparent the "strictly stock" parts were not only not adequate, in many cases they were the cause of severe crashes.
During the 1960's, with the advent of uni-body construction by Chrysler (actually Hudson had used this type of construction much earlier, but before the advent of things like rollcages or even rollbars) and higher speeds, teams of necessity started to build a roll cage which reinforced the body structure. By this time the race cars, even those with full frames, were being build from the ground up, either from a stock, off the showroom floor car which was completely disassembled, or from the component parts.
With the demise of full frame, rear wheel drive cars from the General Motors line up in the 1980's, NASCAR allowed teams to use the sheet metal from approved models in the GM line up, mounted on what had become over the years, a purpose built chassis. When Ford discontinued the Thunderbird, the best selling vehicle in their passenger car line up was the Taurus and NASCAR approved that body style to become the replacement. When Damiler - Chrysler wished to re-enter competition once again, NASCAR approved what at the time was the most popular selling vehicle in their Dodge line, the Intrepid.
With all the complaints from the fans about the current race cars not being stock, one would think that it is only in recent years that this situation has come into being, but those cars have not been off the showroom floor stock for quite some number of years. Forty or more at least, probably closer to fifty.
As for the complaints about the cars not looking stock, all being look alike, etc.; I guess no one has taken a very close look at what's running up and down the highway today.
And the complaints about the aerodynamics; this is just about the only area where what is learned on the race track in NASCAR racing can be applied directly to future production models.
NASCAR certainly seems to have been able to produce a pretty good product to sell to not only the fans, but to the big businesses which provide the dollars to drive the show.
