I’ll have to use the socket in vice grip method.
Even that isn't necessary.
If you go for a "box end wrench
", be aware different makes have different o.d. around the ring and different angles between ring and handle, some of the latter scrape the chaincase plug thread. Amhikt.
Or here's the method I used for a long time ...:-
When turning adjuster screw clockwise to find end of pushrod, hold the screwdriver only between thumb and forefinger. Then, once the adjuster screw's up against the pushrod you won't be able to turn it any further.
The recommended 1/2-turn anti-clockwise to leave some clearance between screw and pushrod isn't precision engineering, approximate will do ...
Fit the screw locknut up to the pressure plate, check 1/2-turn clearance is still present; absent box end wrench
or similar, note adjuster screw slot position then tighten nut with socket and standard ratchet, T-handle, whatever.
Withdraw socket and observe adjuster screw slot position; did it move when you tightened the locknut? If it did, loosen the locknut again, turn the adjuster screw anti-clockwise a little more than before and tighten the locknut again. Observe adjuster screw slot position; is it now in about the right position to give about the 1/2-turn clearance? If yes,
move on to adjusting cable.
Btw, assuming your bike has the standard '70 chromed clutch lever clamped around the left handlebar, be aware the distance between the centre of the pivot and the centre of the cable nipple should be 7/8". There are some pattern levers with greater than 1" centres, these over-rotate the "Clutch lever" and balls in the gearbox case, by pulling too much cable, potentially leading to the balls dropping out of the clutch withdrawal mechanism.