In the spirit of adding more detail/background for this discussion, I was researching the Alfa Bulletin Board forum to learn more about Alfa Montreal owners using the shorter Alfa Spider/GTV clutch slave cylinders when the supply of longer versions dried up from ATE. I found some discussion and warning about using the shorter version and folks tended to refer to a site run by a Montreal enthusiast named Bruce Taylor. From his site, The Alfa Romeo Montreal Website (alfamontreal.info) in the clutch section deep down the page he cautions about some potential issues with the shorter slave cylinder for the Montreal. Not sure if the same issues would apply for the Pantera, but they may, based on Jack’s (Bosswrench) comments in this thread and his recommendation to get as much clutch throw as possible on the Pantera. I pasted the text from Bruce’s Alfa Montreal site where he covers the clutch slave cylinder in detail. Hope it's helpful. Thanks, Mike
“While the clutch slave cylinder (105.64.12.050.00, ATE 03.2525-1901.3) has the same 1-inch (25.4 mm) bore as that fitted to many other Alfa Romeo models, the Montreal version has a longer 76 mm pushrod and a longer cylinder that permits a stroke of 17-19 mm, compared with 11 mm for the version for 4-cylinder cars. The more readily available shorter slave cylinder can (only just) be used as a replacement if it is fitted with a pushrod of length about 70-72 mm. The margin for tolerances is quite small. With a shorter pushrod, the piston will be near to projecting from the cylinder when disengaging a new clutch. With a longer pushrod, the piston may bottom in the cylinder before a used clutch has reached the wear limit.
Replacement clutch slave cylinders with the same length as the original can be supplied by Giorgio Penatti. Classic Alfa can also supply a repro slave cylinder (MN014) of the correct dimensions that is made from aluminium alloy.
Alternatively, the original slave cylinder can be resleeved in brass by brake cylinder specialists such as Sierra Speciality (USD 50), White Post Restorations or J & L Spares. Stainless steel slave cylinders have been made in the Netherlands and are available for EUR 200 from Gert van Kooten. The ATE rebuild kit for the 4-cylinder car version of the slave cylinder is compatible with the longer version fitted to the Montreal.”