Sounds like your clutch itself is fine and you need to convert to hydraulic as others have said?

How much torque does your car make and at what rpm does it peak?

My car made 330hp but 387 ft lb on a t34.48.With a standard clutch cover with an ap 6 paddle centre, still suffered slip but only in 5th gear on the motorway whenever you put your foot to the floor at about 80mph as it was at peak torque.

Going up through the gears no matter how hard you tried it was fine.

Had to go for the heavy cover to stop it