I agree with Psycho, the four bolt valve covers are less common that later styles. The reason they can be considered a little more valuable? You guessed it, most Max Wedges used the same four bolt style! Every big block prior to mid year 1964 used the four bolt style and the bolt on rocker stands and shafts, later big block heads went to cast in stands and six bolt valve covers..
Also, the heads you have are more than likely the small port closed chamber 2406516 castings, these are basically considered low performance and are not much good for serious performance without some serious porting work. I'd swap them out for some 1967 hp 440 heads, casting #2780915, these use the same closed chamber design so it won't lower your compression using the stock pistons in your 413 as the later 2843906 and other open chamber HP designs will do.
These 67 heads are not as common so are usually expensive, changing pistons will allow you to use the very similar much more common "906" open chamber heads or the later model but also very similar "452" heads which utilize unleaded valve seats from the factory.
Good luck!
