At what point does moving the engine back change the cg

[RUSTYNTABATHA]

would like a little more clearence between the engine and the splash well for access to the bilge hatch.. i have a 6 " jack plate and a 14 inch jack plate.. was thinking of mounting the 14 inch to allow more room to open or close valves in the bilge area when at sea.. as well as open check in there... i like to look with my eyes from time to time even on new vessels... i think the 6 would work fine for now but the 14 would really help out.. this thing is light and when used on bass or racing aplications really change the running attitude of the boat but uncertain at lower speed.. we use soccer balls or the old play ground balls as a cheat for extra floatation on bass boats to keep them from swamping when coming down off plain to fast... the ball floats the stern up on swell coming back into the transom..
any thoughts ???