I hear the rod on the 8hp is more prone to breakage than the 5hp because of all the extra rotational mass in the 8hp. If you could find a rod, id probably just throw the rod in there.. de-govern it and call it a day. I think the 8hp is good for huge torque and the 5hp flathead is better for revs. Not saying it cant be done, but it most definately cant be done cheaply.