I've read before that it is advised to install frame connectors with the car either on a 4 post-drive on type lift or on jackstands with the weight of the car bearing on its tires.
I have also seen bare shells on rotisseries with frame connectors being added.
Soon I am going to begin work on a 65 Valiant Convertible. The owner wants some handling improvements including chassis stiffening. It currently is a roller with no engine/trans or interior in it.
I'm curious of any troubles I might encounter by doing the chassis stiffening while elevated on a 2 post lift. I'd think the lack of engine and trans would reduce flex with the car in the air as compared to a complete car. My thinking is that if the door gaps are consistant with the car in the air, it should be safe to install the connectors.
I plan to use 3x3 .120 wall tubing cut and boxed to sit against the floor, similar to those that I made for my brother in laws 72 Duster.