If all your C/G calculations are correct and the plane will require 12oz of lead in the nose to balance, then yes, by all means put a larger (heavier) engine in the nose. I have a G.P. 40 size Stick (a rather old version) and it's wing cord is 12.5". If your cord also measures 12.5" then 3 3/4" is 30% of the MAC. That's a good number. This plane should fly when balanced between 25% and 33% of the mean aerodynamic cord (MAC). 30% is close to the middle of that spec. I have mine set at 29% (3 5/8") and it flys well, and is powered by an O.S. 46 FX but it doesn't weight anywhere near 8 lbs. Double check your numbers and if correct, go with the bigger engine.