Sometimes an engine with down thrust has enough power to pull the airframe straight where the prop disc is pointing. On a high wing, the drag of the wing will "pull back" and the two (the thrust and the drag) finally balance out with the wing cranked up at a higher Angle Of Attack. What does that usually do? Causes the airplane to climb.

If your engine mount is one that bolts to the firewall, it's quite simple to place a couple of washers behind the lower mounting bolts. That reduces the downthrust. You don't need to cut anything on the airframe to try it out.

Very few successful models have over a couple of degrees of downthrust.