[Lynn]

There is another option. Lots of 69 Z/28's out there without the original engine, but the current owner picked up an actual DZ code block that is at least close in time to what the original was. When they go to sell, it is usually marketed as "NOM correct DZ 302". Yes, a little more trouble finding a correctly stamped (NOT restamped) block, and even more trouble for you, being a COPO, but they are out there, and I think that option at least deserves mention. I also like the idea of documenting what you do in some way and secretely marking the block, just in case some crook down the line tries to pass it off as the original born with motor. Maybe stamp something on the smooth surface that the starter mounts to. That way it isn't visible unless the starter is removed, but is still easily accessible.