OK...I forgot about the blackouts for the BB codes and the Z stripes. That makes sense now. So even though the codes themselves don't directly reference an engine combo but the work to prepare a body does indirectly reference what is to be placed in it when sent through the wall.
I always thought the differences between the L35 and L78 was the the shape of the radius rod (round vs. square) but the rod evolution seems to have changed as the production run progressed and the floor plate was the same with either rod so that idea isn't valid. Otherwise I can't see any body preparation differences between the two BB's. I don't why Chevy didn't adapt the 2 radius rod setup like the Firebird.