If the tube lands right in the middle of the frame, then yes. As it is in that Pic, it's spanning the whole frame. I did my Sliders on my 4runner the exact same way, except I didn't do the gussets. I beat the fuck out of them, especially with how low it was and never had an issue. The truck I'm doing now also has the entire exo tied into those Sliders. It won't be an issue.i personally plate the frame where the sliders mount
As far as the debate on the gusset. Anything is better than nothing, but a simple gusset is more or less moving the failure point back. This may be enough for some, but a racing rig, I'd personally go further.
A few years back I read a thread where Carl jantz was building a frame from scratch for his jeep. He explained how he was actually going to make the top and bottom of the frame thicker. I don't know if I can put it in words are good as he did, but think about an I beam. In order for it to bend, it has to compress and/or stretch the thickest part.
My point is that I would plate the bottom of the frame right there, much further back, then add a gusset.