You have a bad heater core.
you don't have a water leak under the dash, do you? that might soak carpets and then the moisture would get blown around when the fan gets turned on.
nice to meet you as well. have you any pictures of the car?
the 63 Special is an excellent choice for an Englisher. the Buick 215ci is
Andy, I think you are talking about valve seals. They were only on 1966 nailheads, and only on the intake valves. All of the earlier Nailheads did not