I used 14 ga. tubing in and effort to find a balance between weight and strength. I think I got it right because I can grab the bar between the back of the rear frame rails and lift up the car with very little if any flex. I did not put plate, just a some sheet of the same thickness because the old sheet was rusted out where the crossmember met the rocker. The reason i didnt put a plate was because I am building a full frame car and supporting the body off of it while I believe from the pictures that you are stiffening the existing structure. Then again the plate idea wasn't completely lost on me, here is a pic where the crossmember meets the rocker. This is an older picture but it does the job.