As the OP said designs, it would be true that many of those designs are still flying today. The 727 and 737 are derivatives of the 707 and the 757 isn't far off. Just different engine/gear/wing/length configurations.
New 737's may have more composites and more modern manufacturing techniques but it's not any radical departure.
For General Aviation, there are tens of thousands of planes from the 50's and 60's still around and the engines they use (Lycoming/Continental) are still based on designs from the 50s with modernized metallurgy, valves and fuel injection.
Yup. Even the fuel injection (in most cases) is based on 1950's designs though.
Incidentally, aero engine makers were playing around with direct injection, electrically controlled turbo waste gates and turbo-compounding at the beginning of WWII.
New 737's may have more composites and more modern manufacturing techniques but it's not any radical departure.
For General Aviation, there are tens of thousands of planes from the 50's and 60's still around and the engines they use (Lycoming/Continental) are still based on designs from the 50s with modernized metallurgy, valves and fuel injection.