getting ready to replace entire floor in a 66 Charger, is the body stiff enough to do so without support from floor or cross braces in door frames ? Want to do on rotisserie if safe, thanks for any advice :-)

you should be ok without supports for a floor pan.
some people will overkill and do it anyway

I wouldn't do a floor on a rotisserie, there's no point, all the welds should be done inside the car.

Last full floor I did on a rotisserie was a 67 GTX, and it worked out fine. With solid rockers you should be safe

Mine was done on the rotisserie, you can tip it on the side, stand up and spot weld it in place through the door.