You shim the top two engine mount bolts so that the nose points more down. That's assuming, of course, that the thrustline is actually the problem. What's also very possible is that the plane is simply out of trim. With a plane that is trimmed wrong for straight and level flight at full throttle, it's possible to glide around at part throttle. I'll make the same suggestion to you that has been made many times on RCU- connect with an instructor so an experienced pilot can trim and inspect your plane and give you pointers on flying it.