Well I have a couple of these. One has a .90 mounted sideways and the other has a Saito 100 inverted. You really should strengthen the fron end both inside and out and its not much work. Instead of actually using glass cloth use a mix of epoxy and Great Planes Milled fiberglass. Its just as strong. Run it along the firewall corners on the outside and the indside and a bit along those stringers. You don't need to apply pounds of it but just simple beads for added strength.
* Also slop some on the inside area of the retract mounting blocks. Y9ou may have to remove them but its no problem. This is one of the smoothest flying and hot P-51s for it size on the market. Another thing about the plane unlike most other warbirds is the front end being so aerodynamic. Everybit of the prop is exposed unlike planes like the Corsair, Bearcat sea Fury etc. Its a mover even with a .46-.60.
One other thing is the elevator is run by one horn on one side if you know what I mean. I don't like it and changed it to one rod from the servo BUT I split it up like a "Y" and run to each elevator.