Figuring compression

Im guessing ( maybe Im wrong ) that I cant just order for instance a replacement piston for a 440 that offers a 9.1 compression because there are alot of variables that have to be considered.

I need to know what these variables are and go from there and yet I know guys are ordering these rebuild kits everyday and stating they have X amount of compression in their engine.

Ok so cam choice does make sense, is that the first step then ?

440 is going in a 4 wheel drive truck, truck is used mainly for show, I am going to do my best to build the 440 for torque.