It sounds like you've got newer heads than the block, or the engine was made post-1959. There are several websites where you can research head and block casting numbers, but there's a very strong possibility newer heads have been installed sometime during the past 50 years. The shape of a machined pad on both ends of the heads can give you an idea of which ones you've got. There were several different valve sizes and compression ratios on 283's over their production run, as that engine was used for everything from big trucks to Corvettes, and there were several engine options with different horsepower ratings on regular passenger cars.
Jerry
