metal fab question

first of all for a bridge that goes between the frame rails, for behind/under axle brackets.. would 1.5" 3/16 wall square tube work or would i need to use 2x2" 3/16 wall?

the bridge will look similar to this, with 5" O.D. tube and circle plates for the upper bag brackets, and will all be under the bed.

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my next question is, i dont have any 5" od tube on hand so will i have no kind of notch the 2x2 or 1.5" tube to butt against the tube, or will it be a small enough gap to just weld?

thanks alot.