I kind of saw both sides. The rear end, springs and shocks in my '66 came out of a Chevy station wagon ambulance a fire department wrecked. The 396 was out of a rust bucket I bought from a neighbor. The THD 400/475 transmission came out of a 14' aluminum body step. Heavy duty radiator was stock out of a Buick. Bucket seats were out of a Tempest. Exhaust was whatever pieces you could clamp to an adapter flange bolted to a set of Hookers. Two matching Thrush glass packs was something to be proud of. Things like cams were purchased from Ed Iskenderian, but a lot was found by combing the junkyard. Sandblasting and spray paint converted the parts to "official" hot rod parts. Not a lot of that with hot rodding modern cars.So modern performance is very different than yesteryear in some ways.
You send files over the internet to a tuner that tweeks the tune.
The aftermarket response is counted in weeks when a new model hits the market.
The modern cars have far exceeded the legendary cars of the 60’s. Bone stock the top dog’s are running in the mid 10’s that was S/S class in the 60’s.
One thing didn’t change you need $$$ to go fast.
Even relatively low priced parts were really expensive when you had a wife, kids and a mortgage and jimmy carter was in office.
The neighbor has been tuning his Mustang GT with a laptop. He has a hot Subaru also.
(warning, old guy story coming)
I had a tackle box with dozens of pairs of main jets for tuning. Take your buddy in the car with a stopwatch. Make a test hit, guess which way to adjust. Take the carb apart, twist in a different set of jets, test again. No data records to print out. We could not have imagined real time onboard air/fuel mixture data, or a computer that could overlay the A/F ratio chart over an rpm chart. Back then a laptop in the garage was a girlfriend that really liked you.