If your seat has a good back rest, your chances for whiplash are greatly reduced. That is the reason car makers started putting higher backrests on cars. Back in the 50's and early 60's, cars seats with backrests that came up to the shoulder area. This caused alot of cases of whiplash which prompted the use of higher backrests to keep the head and neck from over extending during an accident. Even with a four point harness and a high backrest seat, there is the possibility of whiplash and there always will be, even with an airbag. Four point harnesses do help though. On a side note, I noticed that Volvo had a prototype at one of the big car shows of a user friendly four point harness. Volvo was the first to introduce the three point harness that cars use today. I would say that some version of a four point harness is in our future.
As for the strength of older cars, they are stronger than new cars in ways and weaker in others. New cars are built with very strong safety cages around the passenger compartment. The rest of the car is built with crush zones. This allows the car to crush easier in a crash. From the standpoint of repairing the car after an accident, this is bad. From a safety standpoint, this is very good. When the car crushes, it is absorbing energy. The more energy that the car absorbs, the lesser amount of energy that will be transferred to the driver, thus, injuring the driver less. In an older car (someone may want to correct me on this, Camaros may be exceptions), the passenger compartment is not as strong as newer cars. This is where the roll cage helps, (a roll cage also supplies a mounting point for four point harnesses) it greatly increases the strength around the passenger compartment. Because older cars use much thicker sheet metal, the crush less in an impact which is good from a repair standpoint, but bad from a safety standpoint because the car absorbs less energy. That excess energy has to be absorbed by something and it is usually the driver. Roll cages and four point harnesses also help in energy dissipation. If anyone notices errors in my explanation, feel free to correct me as I am no expert on this subject.