I have done both square and round connectors in my F bodies.

I used 1.75x.095 round tube run in an X under the floor. It really firmed things ups, created a natural cradle for the driveshaft, and only required minimal mods for a complete loop. However, as a race only car, exhaust clearance wasn't a problem, which in a street application would be.

Another one I ran 2x3x.083 rectanglular tube in a a more traditional fore/aft arrangement. This also resulted in considerable rigidity in the body. I welded the front plate into the front subframe, bisected the floor in the rear passenger foot well, and tied them in directly to the rear subframe. Clearance was the same as any other type of fore/aft connector. But, in using a size that large, you will have to cut the floor to keep the tube straight.

You could use this material to fab up front rails if you were so inclined. I'd say you gain some additional rigidity over using the stock rails and retaining all the OEM inner fender structure, but I couldn't quote how much. With .125 wall, you are adding a bit of weight. However, after that effort and the subframe connectors, your already a big part of the way towards a full frame arrangement.

I used the stock rubber motor mounts because I ran a small block, but I did use an engine limiter strap with heims. Never had any trouble with blown out mounts. I also had some old DC cast iron iso bushing eliminators, but Gary's are probably as solid and lighter.