The spigot diameter (the smooth bit that sits in the crankshaft) is an important dimension.
The splined shaft engages with the clutch discs and you could change the clutch to suit a different gearbox spline, but you would still need the gearbox spigot to fit into the crankshaft.
This is an illustration of a gearbox input shaft (unrelated to your vehicle model):-
http://www.surfacezero.com/g503/data...ft_resized.jpg