Sliding 2X3 rect tubing into rear frame rail.

I'm in the process of putting 2x3 x .125 rectangular steel tubing between the front sub frame and rear sub frame and I'm installing it vertically and sliding it into the rear frame rail. My question is, since it is not a tight fit inside the rear frame rail, are you guys shimming it with Steel stock or something to make it a tight fit before welding it in place? It doesn't seem like it would be that strong of a frame connector if it is only secured solidly on one side. What is the correct way to weld it to make it as rigid as possible? I already have it going back to the spring mounts and I have drilled holes on the inside of the rear frame rail against the frame connector. I just have not welded it in yet. Advise appreciated, please.