Here's what I think may be happening.
The cars of this era that have a cable-operated clutch use a bell-crank type arrangement to transfer the thrust of the clutch push rod to the pull on the cable to operate the clutch. It is mounted low down on the forward face of the engine bulkhead just next to the brake master cylinder.
The bell crank comprises two arms arranged at 90 degrees to each other, the arms are brazed to the ends of a tube. Over time, the brazing weakens and the angle between the two arms reduces from 90 degrees. This has the effect of reducing both the mechanical advantage and the length of throw, causing the clutch to become heavier and more difficult to disengage.
The cure is to remove the crank, realign the arms to 90 degrees, and weld in a bit more stiffening.
The picture below is of my car at Kevin Vernon's, showing the beefed-up version that he did for my car when he put the new chassis on. The finger pointing at it belongs to none other than SpannerJuggler.