Ok, from what I remember working at the Dodge place. Early 70s the engines where cooking the heads-ex valves from the extra load. To fix this Chrysler made new heads (basically 452 heads) with the small sparkplug and two holes to help cool the valve, nothing exotic, standard new blocks where just drilled and older blocks-gasket where also drilled when retrofitted with new heads.
You can use regular heads on the MH block and MH heads on any block. I have in my possession a big rod- extra holes block with no chamfer at the top, straight out of a MH. Industrial engine is a whole nuther ball game, but a very strong block and very thick bore walls.
Steel crank and block is worth getting.