The coolest buy right now on heads for older engines are cast-iron "Vortec" complete retrofit heads. These heads are advertized as being intentionally configured to fit pre-vortec engines and they do require the updated manifold. I would say that if you are going the other way (vortec block/ older heads) that you will want to examine the water passages on both carefully. Another issue is the possibility of having a combination that is technically compatible and will run, but will have too low a compression ratio.