Hmm, I see the wisdom in both of your responses. I decided to use mbbuna's method as far as placing it (I bought it last night) on the sand bed over a rock to attach, but I want to epoxy it eventually as Julio said.
mbbuna, the thing that I'm worried about is the rock with the clam falling over and disappearing into the aquascape if it's not epoxied. This has already happened with some zoos that I was propigating, and as hard as I worked to get the aquascape I wanted, I'm not tearing it up to look for anything. I was under the impression that unless you're buying a wild clam (mine's aquacultured) and are looking for parasites when it arrives that the only thing that you really need to worry about is something (like a bristleworm) coming up through the sandbed and attacking it; since this won't be on the sandbed after I move it, what other things would I need to check?