906 Heads?

If they are 516s, then you have closed chamber heads. If you're not looking to get the pocketbook out, leave them alone.

The only other factory head that would be a bolt on upgrade would be the 67 only HP 440 casting, the 915. They also have closed chambers, but have the larger exhaust valve and much better ports than the 516.......however, people think they are worth stupid money......so you may as well go with some Edelbrocks or Trick Flows. ....so if you're not lookin to drop a lot of coin, leave it buttoned up.