P515 should be an R3 derivative.
Go take a look and check out the filled in pan rails, main cap support, lifter bore beef and so on. Should be very R3-ish because that's the origin of the product.
The whole "magnum race block" thing is strictly marketing hoo-ha to account for a couple really minor machining variations...provisions for a magnum crank sensor mostly.
I vaguely recall a conversation with Jim S. around 2012 or so, where he said he couldn't get the magnum lifter bore spiders to work. I am pretty sure that aspect of them is not very "magnum" at all.