Th400 started slipping ?

[70GARY]

Just baught a project car. 454-th400 chevelle. No test drive, from ebay.got it off the transport and dropped the hammer. Ran strong for a limited time in the parking lot.. Next morning went to take her home and it slipped somthing fierce. Slowly giving it some gas it slowly picked up speed but didn't shift. Couldn't get past 20 mph. The fluid is at the proper level. Any ideas?