This is an excerpt from 440source.com concerning HP blocks. Interesting.
There is absolutely no physical difference between a block with an HP or HP2 stamp and one without.
All blocks were cast and machined at the foundry, and then assembled at Chrysler's 2.1 million square foot "Trenton" assembly plant in Trenton, Michigan. During assembly, if the engine was given high performance cam and valve springs, or in some cases other items that differed between HP and non-HP engines such as: six pack rods, carbs, etc., then the engines ID pad would be stamped with HP. All blocks originally came from the same place, and there is absolutely no difference in cylinder wall thickness, nickel content in the cast iron, strengthening ribs on side of the block by the freeze plugs (see below for the real story on this,) or any of the other myths you have heard. The number 2, which you may find after the HP, (or in the case of non HP blocks, you may find just a "2" by itself, usually in the lower right hand corner of the ID pad) refers to the shift during which the engine was assembled. In running a factory, one 24 hour day can be divided into three 8 hour shifts. The first shift was considered a "1" however to save time, Chrysler did not stamp a "1", they just left it blank. So an HP2 engine, only means that the engine happened to be assembled on the second shift. There is no advantage to an HP2 block over an HP. we have never seen an HP3, (or just a "3") although Chrysler does allow for it in their documentation, so let me know if you have one or have seen one. Also of note is that non-HP engines usually used the same heads, intake manifold, and many other parts, which is great since it makes it a lot easier to bring a non-HP engine up to the power levels of a factory HP and beyond. But unless you are doing an original restoration, don't worry about finding and/or paying extra for an HP block.
