Unless the block has been machined before, your deck hgt is probably stock, usually about .025. That will lower your c/r a bit more, as the specs in the catalogs and literature usually assume a 0 or near zero deck hgt. The piston choices will always read a bit inconsistantly, each company will write up their products a bit differently. You have to estimate based on what you have now, or will end up with after any machining work. Thats why it is important to know all your measurements before you start to build, or even to buy more parts...your deck hgt, head gasket thickness, combustion chamber volume (dont go by the published figures), all these things will effect your c/r quite a bit. You really have to know where you're at. As far as the headers, I dont know about the length, not familiar with that part of the equation...but I know too big a diameter will move your torque curve way up the rpm range.