Probably a nice block but not really an race "R" block.

From the instructions:

"Magnum Race Blocks
Magnum race blocks are designed for general HP use and
entry level racing. These blocks are a heavy duty 5.2L (318)
Magnum (P5153579) or 5.9L (360) Magnum block
(P5153452)."

Certainly not inexpensive new, Jeg's shows about $3,550. Not sure if they are really available as it says to call.