I like the solid bar idea.I would use something like this.
Square aluminum stock bars.
That is a good thought, and you are on the same page with me as far as visualizing the load and stress points.Steevo,
Just thought of another thing you may need to consider. If the newer engine is narrower the weight and torsion points will not be located where they were originally. Could this cause the mounting plate to flex and bend? Perhaps you may want to put the 3/8" bar on the bottom make them out of carbon steel and run it all the way to the edge of the mounting bracket. You could even stitch weld them on. This would add strength at the outer mounting points along the frame. This is only if you think you will need it. It could also be I am not visualizing it correctly.