[mickmate]

With the 427 especially it's such a big bore with close centre distance it makes the wall very thin. They made the outside of the wall a squared off cylinder to increase strength and still allow coolant between them. Sleeving is not an uncommon practice as the combination of core shifts when casting, and the thin cylinder walls, and old rusty blocks corroding from the back, have the potential to leak or throw a leg out of bed (collapse completely).
Ultrasonic cylinder wall testing is very helpful and advisable before throwing alot of money at a motor. Plenty more advice and experience on it I'm sure.
Cheers Nick