I need to replace the roof skin on my '70 'Cuda. Is the substructure plenty strong to keep things from moving around or do I need to put bracing in while the skin is off?

I am no expert, but if the car is stripped and sitting on the ground, I would think that the roof's substructure would be strong enough. If it still has the engine in it and on jack stands, I will defer to someone more knowledgeable than me.

It never hurts to add support under the car as you work on it but the roof skin on old Mopars is not as structural as you would think when the car is stationary. It should be OK as long as the rest of the car is pretty solid.

If your unsure put the front K-frame and rear axle on jack stands then add RV leveling (Screw Jacks) along the frame rails and rocker panels for support.

You should be okay, but a little prior planning prevents....

The body is on a rotisserie right now. I thought I'd rest the belly of the car on something so the center was supported and rely on the skeletal structure of the roof to keep things in place. The darn vinyl roof made swiss cheese of the skin that's on there now. I've got an AAR and have been thinking of not puting the vinyl back on.....I like the AAR's better without a vinyl top. How much crap am I going to have to take if I don't put the top back on? Everything else is numbers matching.

It should be OK but I would support it. Even without the engine and rear axle etc. you are supporting the car from the very ends instead of the closer to the center of the car the axle & K frame normally support it at. Thats several extra feet of leverage on each end adding additional stresses to the body.

I think you would be fine because the car is a shell but I would not risk it.

Hope it helps.