The Good!! The Bad!! And The Ugly!!!!!

Frame connectors basically tie the front &rear subframes together., essentially giving you a full &therefore more rigid car. Some shops simply use 2x3 square mild steel, insert them into the end of the subframes & weld them. Others will notch them accordingly then weld them. The problem with that (IMO) is that you're decreasing ground clearence along with not necessisarily reinforcing the floorpan itself which is what the subframes are mounted to from the factory.
The best way to add the connectors is to slit the floorpan, put the tubing in 2-3ins inside the subframes then weld the connectors to the subframe & along the floorpan so they are now an integral part of the floor. Once that's done you can weld 2 braces from the frame connectors to the the rocker panels fro additional strength giving you a solid foundation for any chassis mods. Some guys do it at both end of the connectors, but what's important is that they be done where your main hoop for the rollbar is going to go. Some guys will go even further, welding a similiar brace inboard to a 6"x6"x 1/4in plate contoured & welded to the driveshaft tunnel.
Look around here, you'll find pics of how it's done. I think "Inkjunkie" has posted some pics of his car.