2x2 at .083" or a little thicker should be plenty, especially if you tie the roll bar into it. 2 short bars just behind the seats angled in from the main hoop and welded onto the frame connectors help a lot. Tack-welding to the floor where the connectors touches also helps.

It's not always how big or thick the tubing is but how it's put together. Race car fabricators save a lot of weight by "triangulating" the tubing in the chassis/roll bars.

I never cared for "bolt-in" frame connectors or their smaller size.