Torque boxes, subframe connectors needed?

Subframe connectors before torque boxes (yes, they should be welded the entire length). But if you are stripping out the interior to weld in the connectors, you might as well install torque boxes at the same time. I'd bet you'll want the torque boxes eventually, and you'll have to strip out the entire interior again if you wait to do it down the road.

Stiffener plates on the bottom of the lower control arms won't help chassis stiffness, but they sure will help eliminate flex in your steering.