RE: Overloading an engine
As long as the engine is propped to spin above its peak torque RPM, at full throttle, you should have no problems at all.
If you prop it BELOW that RPM, it would lug and possibly detonate (that could damage it).
And when flying at speed; because the torque rises AND load is reduced simultaneously, the engine will 'unload' more than usual, possibly causing the mixture to become too lean...
You don't want that either, believe me.
Oh... You don't know what RPM the peak torque is!? Use a good prop calculator and do the math with known prop loads.
When you increase the prop load, RPM is gradually reduced... At some point the RPM drop will be greater than you expect...
This is because you have gone below the peak torque RPM.