340 heads on a 318?

I've got 1975 318 with the smog heads and cam, and I've been putting together stuff to replace all of it. Got the factory four-barrel 340 manifold and a hotter cam, but I need heads. I've heard that certain 340 heads can be bolted onto a 318 to boost compression, but I've also heard certain castings will lower the compression. First off, is this true, and second, which casting/year/whatever would work to boost performance on a 318? Any help would be fantastic. Thanks.