Hi Timber99, Establish the center line of the firewall. Measure the horizontal distance of the existing mounting holes from the center line. Are they offset? If not, just locate the new engine centered on the firewall. If they are not the same, you will have to get out your geometry book to locate the rear of the prop at the same point that the original engine put it. Hopefully, the engines are the same length from the firewall. If the new engine is shorter and the prop does not clear the cowl, you will have to epoxy a spacer to the firewall. You can live with the new engine being longer. Dan.