There are more than 1 or 2 pictures around of cracked blocks, those were the ones I was able to find quickly from Troy Tribby's 60 Pontiac race car. Troy attributed it to solid mounts and went with a plate system after destroying his engine.
Pontiac knew that there was a possibility of splitting blocks in at least 1970, that's when the redesigned the mounts on the new 455 to spread the load over 2 bulkheads in the block ,#2 and #3, rather than only #3 in the early 2 bolt design. If the Pontiac engineers were concerned enough to redesign the motor mount system they're were smarter than I am. If you're stressing the engine further than say 25% over the intended design you're tempting fate with solid mounts in the stock location, IMO.
As I said, it's his car, he can totally disregard the information I provided if he wants to.