Based on the recommended wall thickness range they provide below for 5.5CLR and 6.5CLR dies, the min is going to be somewhere between 0.065 and 0.083. Thinner is just going to flatten out more and be more difficult to weld. Even on a ladder frame, I'd error toward the thicker side unless you want to spend time reinforcing any structural connections. My brother has built a few manx-style buggies and vw sandrails out of 1.5x0.095 HREW. I've noticed over time lower members like bumpers and siderails end up with dents. DOM would be a better bet IMO.
https://www.jd2.com/model-3-die-set-...und-tube-15-od