If I am remembering this right, the Mopar sprint car engine used an aluminum version of the R5 block but with a different cylinder head, W9 I think. The NASCAR truck engine, which came before the Cup engine was yet another deal.
They were all originally based on the LA engine architecture, piggybacked with new block, new heads, new block again and so on until it eventually ended up as something else entirely. I couldn't begin to tell you what interchanges where.
This is all from memory so it may be wrong.