The design is solid, but you definitely want the gusset because you're putting all the down/up strength back on the stock bolts without it and they're already pretty weak. I did mine the same way originally and thought they seemed plenty sturdy til I started pulling people out of mud and could see the bumper moving up and down independently of the rest of the Jeep.

 

I used some 3/8" plate for gussets on the frame side of my last pickup. Held through 3 deer collisions and a couple of good yanks trying to unstuck people.

If I have the space, I run half inch plate of course the last bumper I built was for a pickup, I've just built bumper accessories since then.

 

So I built the rear bumper a year ago now, then cut it up and did it again when I cut and foldered the quarters

Made a swing out too