Is this a factory bore or .010 and what is a safe bore on a 6.0 block? I am thinking a .030. What would be the disadvantange of boring bigger? I am now needing one.

 

A.030 overbore on a 6.0 block with a 4" strocker crank will give you a 408.A further overbore would yeild additional dissplacement,but could put you into the water jackets.Sonic testing the block is advisable even with the .030 overbore.

 

Whitt is right. Some folks are going 0.060" over but the cylinder walls are reeeeely thin. 0.030" seems to be a relatively safe bore.
I would have the block sonic checked pre AND post. Costs about $75 or so and will help ya sleep better, especially if you boost it.