65 Barracuda no Reverse

Daughters car lost reverse dropped tranny pan and filter was really bad looks like its been quite some time since its been replaced . The servo seems to be in one piece but new to Mopar cars , can anyone advise on what to look for to diagnose this ? I do have access to lift . The car seems to be wants fight between gears when put in reverse and move when put in neutral . 273 with Torque Flight Thanks Bud27

Since the oil is out and the pan is off, the next step would be an air-pressure test. The Valve-Body has to come off for that. But Ima guessing one of the clutch packs is jammed up. The air pressure test will prove it and then you can take the tranny down.