New trans time. Do the clutch/TOB/pilot and inspect the fork/pivot ball while you are at it.
I would not do a used trans unless its still in the car and you can test it before the guy pulls it.
New 3650s are about $1100 shipped to your door after you get your core charge back.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.