The problem now is all the electronic junk crammed into new cars.  That's why every year the sheetmetal seems to get thinner, there's more and more use of aluminum, plastic, urethane, and composites.  Every Mustang now has PS, AC, PW, PL, PM, and then add in heated and cooled seats, etc.  Consider the brakes are larger, as are the wheels.  Nothing has really gotten any lighter.  Emissions controls on the engine, EFI, etc. it all adds weight.  In the '60s 3,500lbs was what a Chevelle weighed.  That was a midsize car.  Today any sedan is pushing 4,000lbs if not exceeding it.  Its actually fairly impressive that they were able to get the GT350 down to that weight.