A heater core wouldn't leak on the drivers side, only the passengers side. To test it why don't you bypass the heater. Is it water or antifreeze on the floorboards? Is it only when it rains?
It's common on these cars for them to leak where the wiper pivots come out of the body. Someone here used 2 tubeless tire valves and cut them off right at the groove where the rim would mount on them then slid them on the pivots to make new seals.
