Ran across one article that R meant runs on regular gas and, P means it runs on premium, I believe this was in the ID stamping areas and not the Block Cast in Casting Number though.
Need some time to think yet.
Can maybe do a grinder test on a standard block and the R Block and see which one sparks more. Regular Cast iron is supposed to have a real short spark trail.
Also can maybe come up with some type of hardness test on the rough cast areas of the outside cast areas of the block, simple sharp ground screw driver tip . . see if it diggs in easier on either of the blocks you might have around. Or some other test you might come up with.
Seem kind of odd that the only engine that comes up on the StockMopar.com chart for the Race Engine is that 318 Cast number you have. Probably some well experienced members that know what up with it for sure.