Could be part of power train instead of part of fuselage. Vibration might be a problem.

* from Jeppesen - Aircraft Sheet Metal

Use 7075, it is stronger

It may be advantageois to put flanges on the top and bottom of the 2 engine mount plates since they will help take the loading between the mid and lower mast bearings to the engine and rear junction. If 1/4" thick then roll corners to approximately 1-3/4" radius. If 3/16" thick roll to 1-1/4" radius. If 1/8" thick roll to 3/4" radius and add additional bracing.

Consider using the primary belt guards, both inside and outside the frame as bracing. They are located close to the thrust vector lines.

Consider using Load CB1120-3, 125#, Item 6393 isolation mounts on all 4 feet of the engine. This will also help in moving the engines further away from the frame, which is probably required if the new frame (not this one) is to be used. An isolation mount may also be needed for the chain guard if the guard is to serve as a structural member.