Sub-frame connectors... Please build them correctly!

Give the OP a break here: He is trying to help and really believes in himself.
I agree that tying to the floorpan adds in the reduction of sheer and flex to some degree, but unless you have access to a 4 post jig similar to what XV Motorsports used to use, how do you collect accurate data to prove your point?
I have installed 3 sets of frame connectors, all hand built using 3x3x.120 box steel. They were all cut along the top side to match the floor pan then reboxed and welded. In my Charger, I added fabricated torque boxes and welded them in at the same time. The frame connectors were welded at each end and stitch welded to the floor. The car was always pretty rigid but afterwards, it felt better. It soaked up dips and bumps like a new car. Most rattles were gone and the car felt much more solid.