The advantages of side mounting are numerous and as already mentioned better fuel tank needle spraybar positioning, no hydrolock problems but there are other reasons too. The muffler below provides better noseover protection with with a taildragger. Don,t laff, mufflers make effective brakes and this is standard procedure on many pylon aircraft to use the muffler for a brake by application of full down after touchdown. Now for the most important reason and that is safety. Most engines when mounted this way end up with the neecdle valve straight up and far, far easier to adjust from behind the wing. While I have airplanes with engines mounted numerous ways when it comes to my multi engine stuff I will only use side mounts and it is for that reason. Only safe way to operate multiple engines is with that needle straight up (and with out the cylinder in the way).

John