AB you can make your bracket the same size as the height of the chassis RHS also if you are going to mount the sliders to a flat bar section i would Oxy cut a hole into the flat bar section an therfore would insert the slider half way into the flat bar section as you will get ample weld to the piping section bear in mind when welded properly a 6mm CFW ( Fillet weld ) an inch long will hold 1 tonne.

Also if you have enough clearance you can drill holes into your chassis and mount it that way id be using a M12 grade 8.8 bolt as this will be substancial for that purpose also installing that gusset will be the best thing you can do as this will stop the slider from bending if you get stuck on rocks.

If you are drilling holes especially for things that are going to cop a bit of punishment from time to time and to stop the holes from tearing from each of the hole edges should be a hole and a half to the center of your bolt hole

Also i dont know what you will be welding it with but if you have a stick welder stay away from a GP ( General Purpose ) electrode as the weld is not as strong as a Low Hydrogen weld and personal preference for a welding rod would be a WIA 7016 3.2 or a 2.5 mm rod - Amp range for a 3.2 would range between 90 amps to 120 amps and a 2.5 would range between 65 amps to 90amps.

If you would like any information into the regards of steel fabrication just ask me and i will try and help you out the best that i can ( Boilermaker / welder by trade )