If the bolts loosen (not back up) after each flight, check your prop properties as well. If the wood is not dense enough, it will do that. Choose dark colored wood when buying a prop. Carbon props are better and keep the initial torque quite long. Still need to be checked every now and then though.

The 58 will power my 40 lbs piper cub with ease and power to spare for towing. If you want to fly 3D, think 20 - 28 lbs. The prosport is about10% more powerful.
Tip:
MVVS currently has Limited Edition engine on sale, (prosport engines with special Black/red anodize)