Someone asked ealier about the tubing thickness on 2x2" box subframes, and my reply was:

Here's the way I look at it, and as always, I could be full of it...

Obviously the OEM frame material is a lot less than .120 wall (1/8"), so that begs the question, why use anything THICKER than the existing frame? Well, the frame sections are a lot taller, AND they are tied into the floors/fenders, so they are "borrowing" load-bearing capacity from adjacent structures. That's the beauty of unitized construction. In contrast, the subframe connector is mostly on its own. It's tied into the rear floor for ~1 foot, and then it's just hanging out there. In that case, it helps to have it a little thicker. May want to look into tying it into your seat mounts that come off the rocker panel. Pretty easy to do.

I used 2x3x1/8" on my Charger and I'm currently installing 2x2x1/8" on my 71 RR.

If the car is going to see much track time, I'd go with 2x3" tubing, and with the 14 guage (thinner) material like M D and Andy recommended.

I'm going with 2x2 11 guage on my 71RR because 1) it's a street car and 2) I don't want the connectors to be visible in the front seat area. Too bad my pic hosting service dropped me (moparpages) or I'd post a bunch of pics.