The reason for the t pieces is that unless you weld tubes into the box section where the holes are - there is a possibility the the box section can collapse when you put a bolt right through and then tighten it up
Does that make sense ??
Hense the reason for the brackets (+ I welded the box section in before my seat mounts came and non of the holes on the side mounts lined up with the 2 rails I'd welded in !!!)
Dave