My 76 302/C4 does not have a convertor drain plug. I recently bought the car under similar circumstances-little old lady one owner car, 58k miles, fluid never been changed. Had to do multiple fluid changes, new modulator valve, clean valve body and governor valve, and shifts perfectly now. These are not hard to do and inexpensive if you want to do yourself. Haynes has an excellent manual ["Techbook'] for about 15.00 which covers Ford transmissions in detail with good pictures. No guarantees that this will work, but may be worth a try before springing for new/rebuilt trans. Good luck.
