Hi Chris,
Just for your information, I have been flying my stock Phoenix and SuperPhoenix prototype for over one year now with the Merlin 110 GE and P-120SX.
The Airframe handles the power just fine. Top speed recorded at 420 km/h ( 260 mph ). So throttle management is essential to comply with the AMA/FAA speed limit.

The case of the P-120 SX is a bit more critical because of the weight of the engine and the g's it can handle. I'd recommend you to put two vertical hard wood plates inside the fuselage along the engine mounting location to join the lower and upper skins.
This will be sufficient to take the loads.