That's actually a very small chamber for a big block. Normal open chamber big block heads are in the 90cc range. Those are closed chamber (516 castings probably) and in the 78cc range if memory serves me correctly.
516 heads are 84ccs from the factory. Small exhaust valve, but that can be fixed. 915s are 79ccs. Also that single plane manifold with log exhaust manifolds aren't helping.
