I welded them on using 6 jack stands. before flippping it over to do the rest. But here are some observations from another mopar guy.
We get asked the question about installing the frame connectors with and without the weight of the car in place a lot. There is no doubt that a unibody will sag a bit when you hang all the parts on it (doors, glass, engine, trans, rear etc.). This seems to be more of an issue for getting the panel gaps correct i.e. if you hang the doors & fenders, then install the entire front end, K-frame, engine & trans, the door/fender gap will tend to tighten at the bottom from the unibody sag - just a little, not much. One of the by products of adding sub frame connectors is you will make the unibody stronger and help eliminate some of the sag.
In our shop we install the frame connectors all the time with cars on one of our rotisserie's. It is so much easier to work with them NOT upside down! If there is any concern about a unibody not being strong, we tack weld the connectors into position with the car on the ground (did this on the '70 Superbird that was totally a together running/driving car when we installed the connectors!) After tack welding (a lot!) we put the bird in the air and did the finish welding upside down. Not as much fun, I can tell you!
So, if the car is a bare unibody shell, I do not see any reason the frame connectors cannot be installed with the car upside down. If you think the structure of the unibody has been compromised (lots of rust, quarters currently removed, frame rotted off etc.) then some temporary bracing should be added. We routinely use door braces on convertibles to keep the unibody from flexing.
It has been our experience that regular cars (with a roof) that are structurally sound can be rotated upside down and have frame connectors installed without any resulting issues during re-assembly. Same is true for convertibles, we just add the door bar to add the structure back in.
As an extra precaution, you could weld in some temporary braces inside the car to hold the unibody in alignment while the car is rotated. We did this on a convertible recently when we had to remove the A-pillar to rebuild it. We welded braces to the windshield header to be sure it did not move at all during the surgery.
One of the fellows in our shop is working on his 69 Coronet R/T and the rear frame was so rotted, he added extra braces to hold the car together while on the rotisserie.
So, the shorter answers is; Yes, you can rotate a sound unibody upside down and weld in the frame connectors without creating a problem. And if you have concerns about your unibody twisting, tack weld the connectors into place before rolling the unibody upside down for the final welding.
