Sorry, the linkage from the arm to the pedal pivot is the only adjustment there is. If the clutch has started slipping chances are really good that the disc is burnt and the pressure plate has lost its spring tension. You will end up having to replace the assembly. When you do, be sure that you have the fly wheel surfaced so you don't end up burning the new disc out because of hot spots or uneven spots in the fly wheel. Below is what it says in the book about clutch adjustment. Thanks
Fig. 1: Clutch pedal free-play adjustment - 1970-72 vehicles
Only one adjustment is necessary to assure that the clutch operates efficiently. This adjustment is for the amount of free clutch pedal travel before the throwout bearing contacts the clutch fingers. The pedal should be adjusted at periodic intervals to provide 3/4-1 in. of free pedal travel.
Disconnect the clutch fork return spring at the fork.
Loosen the nut (A) and back it off approximately 1/2 in. from the swivel.
Hold the clutch fork pushrod against the fork to move the throwout bearing against the clutch fingers. The pushrod will slide through the swivel at the cross-shaft.
Rotate the lever until the clutch pedal contacts the bumper mounted on the parking brake support.
Adjust nut (B) to obtain 1/4 in. clearance between nut (B) and the swivel.
Release the pushrod, connect the return spring and tighten nut (A) to lock the swivel against nut (B).
Check the free pedal travel at the pedal and readjust as necessary.