Is your clutch hydraulic operated? Brake fluid from a master cylinder? I assume it may be as my 1963 C60 with a 327 engine is hydraulic.

Are you familiar with the term free play? Where the clutch pedal should push easy for a bit before the throwout bearing starts pushing against the pressure plate. If you have no free play the clutch will slip. Probably not your problem but I thought I would mention it.