Try putting the pivot (bushing) on the engine end of your truss, as opposed to putting it on the frame end. The frame section height is greater than the effective section height included at the engine (whatever the vertical dimension is between the holes of the plate you have bolted to the engine. In other words, I'd rather lever off the frame rail (bushing near engine), with a larger gusset to fight twisting, than lever off the engine (bushing near frame).
Here's a coupla pics of my setup as it was being fabricated:
http://home.4x4wire.com/randii/motor/P3290022.JPGhttp://home.4x4wire.com/randii/motor/P3290026.JPGhttp://home.4x4wire.com/randii/motor/P3290031.JPGhttp://home.4x4wire.com/randii/motor/P3290033.JPGhttp://home.4x4wire.com/randii/motor/P3290039.JPGThese may help illustrate what I mean about the taller dimension.
I can't see you motor mounts enough to really say, but try to use them in the plane for which they were designed -- if you're shearing across them at 90 degrees to their design direction, that could be substantially weaker. Ideally, you want to load most rubber/poly more in compression than shear. The tubular bushings as used on leaf springs are pretty resistant to most forces not parallel to their central hole, but I like the Advance Adapters donut style better for their ease of alignment and assembly, if you can cock you mounting surfaces to be more or less in line with the direction of rotation that the motor wants to swing through when it torques.
I have also considered connecting the long frame mounts on my setup with a removable truss, and may add that later if the frame rails show any signs of 'springing' over time.
Randii
