IMO, prep/paint the body of ANY car on the frame and ready to go....so the stresses of loading, handling, and any stresses about mounting on the frame, are there, and hopefully not too much when the car is driven after painting....frankly I would put some miles on the body/car in primer first, so I know damn well it should be fine....I mean over rough roads...beat hell outta that thing....better to know NOW, in primer, when it's easy fix, than a grand+ worth of materials later and having to redo something....
I remember a few years ago, doing a frame replacement on a really nice yellow '72, but there was allmost no way of supporting that nose enough for the paint to not crack near the cowl, for instance....
i'ts a bitch to paint with the body on frame but a bit of tapeing and it's fine....strip all interior. and other items......obviously....