It can be installed after paint as this thread exhibits. So removal is also possible. It would just take some care and involve a little more risk of accidental damage. Modern urethane paint is more flexible than older paint systems so it will take some panel flexing w/o problems.
The making of a 427 Narrow Hip Cobra starts now!!
Bonding and riveting the body isn't any big deal after paint. In the above thread, towards the end, he discusses a little variation on bonding the rocker panel area to the frame member that would help with a painted body.