1966 A904 loses all gears overnight

Drainback is a short term problem, meaning once it runs for few minutes the converter will fill again and the transmission will engage and shift normally. Neutral range is where the fluid circulates best through the trans (unmodified torqueflites only). If this runs several minutes in neutral and still won't engage, the clutch packs are likely smoked, meaning time for another rebuild. This is likely the case since you also said fluid looked dirty. If fluid was brown or black, it's toasted.