Billy, drain the whole thing. I normally loosen the pan bolts and let it drain from the gasket, then remove the pan completely. A 'dry' Cruise-O-Matic holds about 11 US-quarts. Re-using trans fluid is NOT a good idea. Any amount of dirt can ruin a good transmission.
Over here, an overhaul costs about US$1,500 IF you bring it to the shop. Much more, if they take the trans out and put it back in because the car ties up a hoist while the trans is on the bench. Otherwise, the car must be pushed out of the bay, parked for days, then pushed back in when the rebuilt trans is ready. - Dave