I'm welding my loop to the frame connectors. I've thrown driveshafts at 8,000 RPM on the track and the body sheet metal is no match for it. Unless of course you desire a custom, floor-based air conditioning system.
The bolt-on version Gary mentioned from U.S. Car Tool attaches to the crossmember. If available and fits, seems like a good program, as well.
Just my 2 shekals
Jim