Some of the problem is the mechanical part of the clutch release linkage. ALL of it is unhardened-steel-on-unhardened-steel, never lubricated and each little wear clearance is additive. The L model over-center linkage under the dash is particularly susceptible because there are so many steel on steel pieces. After 50 years of this, the clutch disc simply doesn't get enough motion to fully release.
I use a constant-contact throwout bearing that need NO free play at the release lever, which helps. A long throw slave might help, but go thru your linkage and see if there's anything grossly worn. Take that part off and weld up the worn area & redrill. Or add a bronze bushing (or four)..