Ole Car Man

I need to know how to stop the water from leaking inside the rear-floorboards when it rains, the top doesn't attach for about 8", from the window back, and when the top is up, water runs down and floods the floorboards? I had to take two tops (parts) and put together one top that worked, ahd a new top put on the rails, but, the shop had never seen a 1964 Plymoth Valiant before, so could use some tips?
Thanks Kelly