If it has been sitting awhile then all the fluid in the converter has drained back into the pan which escape through the overflow tube.
First thing I would do is get under the car and have a good look at the transmission, look for area that are clean (side of case) that will give you a good indication where the leak is coming from.
It is time for you to drop the pan and drain the fluid, and also drain the converter get all the old fluid out and put some new type F fluid in. Put a new filter on and gasket (make sure the pan is flat where the bolt holes are). replace the o-ring for the filler tube if you detected any fluid leak.
Once you have re-torqued the converter drain plug and the pan is bolted back (criss-cross pattern), start adding fluid (check for any leaks), After you have added 5 quarts start the car don't rev it. Once it is running slowly shift through the gears, (at first you will not feel the car go into gear) let the car warm up and add another two quarts. Go back in the car and shift slowly through the gears this time they will engage. Slowly add more fluid remember not to over fill, every now and then look under the car for leaks.
If you take it to AAMCO they will tell you it needs a rebuild.
1966 Hardtop - Bench Seat
289 CID -4V
C4 - Automatic
Silver Frost - Black Vinyl Top
Los Angeles CA
April 06 1966