There are three main areas to check for leaks in the front area of the car. You need to check all three areas separately and in order.
1. Check the cowl for leaks. Pour some water directly into the cowl, making sure not to get water around the windshield area. The water should drain out each side behind the front wheels. You can also look into the cowl area with a flashlight and a hand held mirror.
2. Check the windshield seal. Run some water around the windshield seal, making sure not to allow water to run over the door jambs and door glass. This is probably the greatest cause of leaks into the front of the car. A 35+ year old windshield seal is probably rotted and no longer weather tight. Also, if the windshield has been replaced or pulled recently, after market windshield gaskets are notorious for leaking - make sure to get the FORD seal if you replace the windshield seal.
3. Check the door jambs and door glass channel. Run water directly on the doors, especially allowing water to run down the glass and into the door itself. There are drain holes inside the doors that can get blocked.
Cover the floor boards with some newspaper (it will show even small drips of water very easily) and check after each test.
Tim