Sliding 2X3 rect tubing into rear frame rail.

I did it that way for strength. The frame rails of the car don't lie horizontally they stand vertically. Also, I can weld more floor into the frame rail this way. I saw pictures on here of other guys doing it that way but it never said specifically how it was welded inside the frame rail. It is just such a sloppy fit side to side when standing it up inside the rear frame rail. I don't have any steel plate material to cut and put in there at home. I will just have to use what I can find and weld it to the outside of the frame connector before putting it in and plug welding it in place unless somebody else chimes in of a better way.