An internally leaking master cylinder would give you the symptoms you describe. Sounds like insufficient travel at the clutch.

Could also be wear on the clutch pedal pivots or master cylinder clevis attachment. But I vote or a worn master cylinder.

IF you have access to someone else's 1800 w/hydraulic clutch, you could measure the slave throw and compare it with yours. If the throw on yours is the same as the operating one, the only things left is a too thin clutch disc/pressure plate combination (but that should be capable of being adjusted out), perhaps a worn clutch fork or fulcrum ball (I had a similar problem on my 77 Datsun Z and it turned out the fulcrum ball had broken and the clutch fork was pivoting on the bare threaded bolt...the PO had adjusted it out as far as it could go rather than replace the fulcrum ball.