On the subject of engine mounts, I have never believed that I, or anybody else, could drill proper mounting holes after the plane is assembled.
My style is to drill the holes one at a time on a drill press, before assembly. Drill the first hole, bolt to the engine and then drill the second hole.
Then, with the engine bolted to the hardwoods, fit the hardwoods to the center block. An accepted combat technique is to add a vertical 6/32 bolt through the hardwoods and the center block. This will give you both correct holes and the ability to position the engine where you want it.