I've had many planes with this type of mount, and while they are "the best" they are fine for 40 sized planes like you have. Don't worry about getting it perfectly centered as it's not going to make that much of difference. "Eyeballed" centering will be just fine here. Make sure the bolts are tight when you install the engine, and if you don't have self locking nuts make sure you use locktite on the nuts to keep them secured.

Ken