I agree that 3/16" seems unnecessarily thick and heavy. No matter what material or mounting method you use, be sure to use gussets above the legs at the mounts.

I'm a fan of having the pinch weld 1/2"-3/4" or so above the slider legs to provide support above the sliders with hard hits. That just limits the amount of flex by the frame and sliders. Any less than 1/2" clearance would mean you'll occasionally hear vibration or contact. Your idea for a 1" square tube for spreading out the impact area on the body would go along with this concept well. Seems like overkill though. They won't self-clearance if you put that tube there, so err on the side of extra clearance. I have semi-circular dents in the pinch welds above the legs, which is fine by me.