<VV> Clutch; transmission, differential diagnosis

[Bill Meglen]

I am preparing to pull the power train on a newly acquired 1963 Monza 
Spyder (everything is disconnected in preparation to dropping it) 
before doing so and tying up his hoist awaiting parts,  I want to 
research if there are other alternate diagnoses than the clutch.

Engine runs car had been sitting for years, driven infrequently if it 
all. PO had been driving it with no prior evident slippage or erratic 
behavior before the PO shifted down coming to a stop sign 
(aggressively?) and the clutch pedal level dropped. The car behaves as 
if the  clutch is disengaged.  Cable had come loose from bell crank.  
Examined it and replaced the cable it checked its movemement. Clutch 
pedal returned to normal height but requires little foot pressure to 
depress it. Same feeling and range of movement at the clutch rod with 
cable disconnected. Engine runs, no unusual noises, but when placed in 
gear wheels do not turn. No evidence that anything in the transmission 
is turning. My conclusion is that clutch is somehow locked in a 
disengaged position and will require power train removal to repair it. 
I presume that replacement of all clutch components may be in order. Is 
there any other likely explanation for this behavior that might be a 
transmission or differential issue and entail longer downtime awaiting 
parts?

Bill