Not trying to get into a Peeing contest with anybody, but anyone that says welding on anything with electronics without unhooking the battery isnt a problem, just hasnt ever burnt up $10grand worth of computer boards. I had it happen, twice. Of course that was a specialized piece of equipment and not a farm tractor. Just a fact that all the electronic control modules dont like electrical discharge. If you have a old 8n tractor, you can get away with not unhooking the battery. This new stuff with all the electronic control modules, you might get lucky and you might not. Hooking the ground as close to whats being welded helps, but isnt a guarantee. Unhooking the battery dont take much time and can save you hundreds in repair cost.
As for welding the expaned metal to the rops, not a good ideal. Making all those tack welds along the edge of the expanded metal will warp the rops. I would make a frame out of angle and weld the expaned metal to the angle, then bolt the angle onto the rops. this way if yo want to take the guard off, you just have to remove a few small bolts.