Have a bbc that is closed chamber and looking to buy a set of aluminum heads my guess putting open chamber on a closed chamber motor will drop the compression . pleas school me on this.....

You probably know this but your piston selection has to be compatible with your heads.

How many cubes, and what are you hoping to get from it?

I'll assume it's a 427, and having not researched the combo lately, I'm thinking closed chamber aluminum heads are fairly scarce?

Do you know what piston you've got in there now? (Any markings on the top)?
What bore size or CID is it, and also what head are you currently running on it?

As Bruce & Dave pointed out: 1. Piston configuration is critical, because many open chamber heads will contact some of the common closed chamber pistons. And 2. There are not many (maybe none?) closed chamber aluminum heads currently being manufactured. You may be able to find some vintage or NOS GM Closed Chamber aluminum heads.
I don't recall details or P/N's, but I believe the Open Chamber aluminum heads Chevy put on the 71 LS6 used the same domed pistons as the 70, which was Closed Chamber. So whatever head casting the 71 used, may work on a closed chamber 454 type combo, if the piston being used is the same or similar to the old Speed Pro (TRW) slug, (thinking it was the LF2307A, 12.5:1 CR on the 70, 11.25:1 on the 71)..

Edelbrock/GMPP heads have some of the smallest combustion chambers but require pistons that are specifically made for them, their squirrely chamber doesn't work with many domes of either variety.