Block casting number search

Im at the end of my ability to search casting numbers. I have what I believe is an R block. Has a plain R on the front of the number 2 cylinder and an R3 by the bell housing at number 8.

The rest of the block makes ZERO sense.

It's 340 mains, 9.600 deck, 59 degree lifter bank angle but has the bosses for 48 degree LBA.

The lifter valley looks exactly like a magnum, complete with the tapped holes for the HR spider in the cam bearing bulkheads. I know the decks are 1 inch thick, the pan rails are all filled. The only number cast into the block is a P number and I can't track it down.

I suppose this could be a 340 replacement block, but I can't find a P number for that either.

I want to know exactly what I'm working with before I start on it.

Here is the casting number.

P4532435

Any info is much appreciated.

TIA