If you can see where the old firewall was, use those locations for the new one.

If there is no indication of where the old firewall was, install the new firewall at 90 degrees to the fuselage (no side or down thrust ).

Make sure your wing incidence is correct as it will affect the flying tests. The incidence should be 0 degrees on the horizontal stab and 0 degrees on the main wing.

When you fly the plane, if the plane consistantly pulls to the left, put a thin washer under the left side of the motor mount (when looking forward ).

Now fly the plane at about 2/3 throttle. Suddenly cut the throttle to idle. If the plane suddenly jumps up you need some down thrust, if the plane suddenly drops down you need up thrust. For down thrust you can either put a thin washer between the motor mount lug on the engin and the motor mount or a thin washer under the top of the motor mount.

Repeat the above flying/washer tests until the plane flies level when you cut the throttle and does not pull to the left.

A variety of thickness of washers is available in the fastner section of your local hardware store (Ace, True Value )