I've been told a 383 can be bored up to 90 thou and a 440 to as much as 80 by a very reputable builder in my area.(I've never taken one more than .060 myself) Obviously availablity of piston in that diameter is an important issue to consider but knowing how much material is actually there gives an idea of how safe it really is taking a B or RB block to that oversize. Many people tend to think .060 is leaving very little 'meat' there.