I also am getting ready to start building a set of sliders.....I will be using 2x4 box tubing for the rail.3/16. I have some scrap 1 1/2 1/8 inch square tubing i am thinking about using for the supports to the unibody. is this strong enough, i'm also using angle iron for the pinch seam.

 

You don't need angle iron for the pinch seam. The 3/16 welds just fine to it.

I'm planning to use 2x2 3/16 to brace with. I wouldn't skimp on the thickness of that tube since it's more in rock territory than the rest of the slider. And if it deforms it might pull your sliders and deform the body. Worst case scenario for sure but the trail tends to find and exploit those

 

for the supports to the unibody I'm planning on doubling up the 1 1/2 to compensate. I should be starting the build this weekend and I'll post some pics. Thanks