i wouldn't use balsa as standoff material, use arcraft ply or a nice piece of maple, 10-24 bolts would be ok , 1/4 20 is what i use ................. i would not use 6-32 , they are too small IMO, thats what i use on my small glow stuff
An idea that has been suggested is to use the H9 4-40 titanium servo pushrods. Seriously light, but geesh, they look kind a spindly :-)
Who ever told you that is full of ****, using4-40 regardless of what its made of to mount a big gas motor would be foolish