AlphaCat will be getting a set of homemade connectors soon (sometime this winter). My buddy put them in his 71 Mach and you can barely tell they're there. We used 2x2 square tubing and kind of semi-inletted them into the floor of the car. They don't intrude into the front floors, but do intrude into the rear floor a bit. After the carpet is back down there is just a slight 1/2" bump in the rear seat carpet! This way we can weld them to the front floor and through the rear floor (notching the floor to recess them in there). The notch in the floor is then welded to the frame connector. It is easier to show in pics than describe so I'll post pics when I do them to my Cougar... They're almost unnoticeable from the side of the car. The best part is they look like they belong there instead of the aftermarket 'hanging-down there' style.
It is real nice to have a buddy with a plasma cutter and MIG welder!