Yeah they have a chance to last another 50 years just as well as the rest of the car. The difference is these cars are pampered now and many live in garages and are driven in good weather. They were abused when they were children.
I really dont see any negatives on adding the plates. Another thing I will note is to be careful lining up the slot for the tortion bar adjuster. Make sure there is plenty of room to get a socket on it. If your doing home made it needs to be a slot because the T-bar bolt pivots so the adjuster wont always be centered if you just do a round hole.
PPSI, what did you use to make the slot on yours? Greenlee knockout punch?