Need advice on frame connectors

The 2 x 2 tubing will work good. That's what I used on my Duster. As for car position, I think that either on tires or on stands is OK. Just get high enough to weld and work on it. Just level the frame/rocker panel to keep it straight. If you don't have latches in then have doors shut and check gaps to be sure aligned right. I did mine on a floor hoist so it was easy to work on, and it still aligned body panels and doors. Engine & trans out is OK.