Subframe connectors for a driver/cruiser, necessary to weld to floor?

I was thinking I would buy the steel 2x2 and fabricate them, just welding the ends to the subframes. My question is really whether or not welding to the subframe without welding to the floor makes sense.

I’m honestly not going to remove all of the undercoating and my interior, but I would like to add some strength.
I have seen them to the floor, there is still enough flex to cause seperation, seen it happen