John,
Just quickly read your postings to get a feel of what your up to.
For the frame start with a depth of section equal to 1" per linear foot of distance ( sorry you can convert ) then
check for deflection or work backwards and find the section based on total anticipated load and see what
I,S,R you need. Stiff doesn't mean resonant free. Beam Boy is fine but haven't used it in years for static loading. From a resonant point of view, nothing is better than weight as that drives the value down and you should have basic design done before you / or if someone would do analyzation. You weld up a frame, besides distortion from welding which you can kind of control ( Hood welds i think ) , after your are all done and if you machine without stress relieving it just can / may go wherever to some extent. Maybe dummy pieces on top of the frame which can be machined post attaching to frame fabrication may be a better choice. The piece is rather large so machining is not going to be cheap even for a few facing cuts.
You can analyze for motor requirements / torque required using a number of free programs. Mitsubishi, Superior Electric, Parker, and Danaher Motion all have one just to name a few.
The first step to getting a good sewing machine to have the wife hem the pants before staples, chewing gum, or crazy glue is used instead of thread.
RICH