The 31 and 35 have the same bore which is 39mm. The stroke on the 31 is 26mm and on the 35 it is 30mm. I don't remember if the wrist pin location on the 31 is the same as the 35 or not. I do know the 31 piston is taller than the 35 which would make more compression but you would need to mill about .025 off the top so as not to "kiss" a valve. All this hot rodding is fine if you need more RPM like the boat guys run. In our use for this hefty engine we are looking for useable torque and lower RPM. I think av8tor is on the right track where he is. Just my .02.