A racer who once ran Showroom Stock in the 80's once told me, no showroom stock car has ever won a Showroom Stock race. Anytime there is "factory support", the cars are a long way from what was sold to the public. Granted, they are modified for safety equipment, but there were plenty of shenanigans to improve the cars performance on track.
The HSR and SVRA rules for these cars are more liberal than the rules for the original series. Some things that were required in the original series are not allowed. When I started running my SVO in these classes I tried to keep it as stock as possible, only to have tech make me do things like install a fuel cell and remove the carpet, all valid safety items for sure. I know the guy that has the original, restored SVO that my car is modeled after and I dare say that if it showed up at a HSR or SVRA race to compete, it would not be allowed on the track. More importantly, in this day and age and from what I have learned about the hideous handling characteristics of an unmodified Ford Fox chassis, along with the limitations of the brakes, I wouldn't WANT to put it on the track. The more liberal modern rules allow for a panhard bar, decent bushings, stiffer spring and better shocks, sway bar mods, chassis stiffening items, caster/camber plates, race brake pads, et al. None of those things would have been legit in the original series, unless they were hidden or specification games were played.
The most significant restrictions in the HSR/SVRA classes cars relate to the engine, trans, wheels/tires, and aero. Even so, the usual hot rod type mods like headers and minor porting, oil system mods and such things are allowed.
There are too many different cars and race series for the sanctioning body to police to make any sort of "leveling the field" possible. Do I care? Not in the least. That is the beauty of this sort of vintage racing. You are provided a well run event at a great venue, put in a group where the cars have somewhat similar lap times and are cut loose to have a ball. I can be pretty sure that there are going to be cars in my group that qualify within a second or so of my time, and that gives me someone to race with/against, even if they are not in my exact same class. That's where the joy comes. After all, there are no purses, you might get a medal or trophy, and a hat. Bragging rights if anyone wants to listen or care. These series are about fun, at least for me, but I believe it to be true of the vast majority of the folks that I have met at the track.