I've had mine for 30+ years too and have absolutely no intentions to sell them plus they are willed to my grown children. Until my demise I will continue to enjoy them. I don't care if the market drops out on collector cars...and maybe is should because greed takes over like everywhere else in life and ruins it. All I have for documentation is an UOIT for one car and nothing for the other. And there is plenty of false documents that look damn authentic these days to make original ones a moot benefit. But, as far as the 'doubting Thomasa's' go, I don't have to prove anything because I know my cars histories and doubters will always be just that. If they don't believe what I have is real...well then they can read my license plate
Don't despair....enjoy them while you still can. Fifty years from now, who will care?