As pointed out earlier, there is no differance between a HP block and a six pack block. People always said HP2 denoted a six pack block, but that just meant it was assembled on the second shift. I read an article many years ago where they interviewed a Chrysler engineer who was part of the perfomance program. He stated that the HP blocks were visually inspected for minimal core shift. He said they would look at how many HP engines they needed for production requirements that week, look through their available blocks, visually inspect them, stamp them HP and segregate them. The block in a lo po Imperial may have met the standards for an HP, but if it was not inspected because they had already picked their quota, it was not marked. Based on the production standards at the time, I believe this to be true.