The 427 truck block already has a 4.250" bore, but I wouldn't take it more than .060" over, the cylinder walls aren't that thick. As for the 366, it's a waste of time and money. The cylinder walls are way too thin to bore more than about .030" without running into problems. Also, the 366 CANNOT be bored to a 427. They are also somewhat heavy compared to a standard deck big block. You're better off finding a builder 427 or 454 from a passenger car or pickup.
Mark