340 Pistons with 10.5 CR with X Heads???

If you do not already know you have to do all the math as mentioned above.The factory 10.5 engines were really around 9.5 actual compression when you tear them down and measure everything.There are alot of factors such as gasket thickness,deck height,piston and head cc etc that are need to know things to pick a piston for a specific compression.Wih all that you should still be able to achieve the compression you want easily with some keith black 243's and either slight block mill,head mill or both and or playing with gasket thickness,all easy to do.