I have owned two classics, a 73 and an 86, and haven't had an issue with the roofs leaking water because of the factory design. What I would pay attention to is any holes cut in the roof or rivets replaced and make sure that they are sealed properly. If there are any areas that have old sealant, I would suggest removing all the old sealant, cleaning the area, and re-sealing.
Jon & Chris Everton
1986 40' Dog House #86
0 hp 8V92 Allison HT740
Soon to be 500hp ISM with ZF 6 Speed