Re: 390 HD ??? What is it?
I have a 66' Fairlane GTA with a 1974 390 V8 truck block. It has a mirror image 501 on the front of the engine block. It is supposed to have more casting support in the bottom end, higher nickel content, and supposedly thicker cylinder walls. Can you see a casting number on your block? If it starts with a 'D' it is a 1970 series. The second digit is usually the year. Example: D4 would be 1974.
Hope this helps!