The spline size (OD) on my GB appears to be 0.735" (kickstart and gear lever splines are the same).
There can be quite a lot of side play at the cable end of the lever. You'll have looked at the gearbox exploded views (eg on Drags site?), the lever fits in a fork on the end of the push-rod extension that fits from the kick start case end. You might be able to check that it's in the right place with a torch through the filler cap hole?
Use the adjustments (screw thread on pivot of clutch operating lever and the cable adjuster in the gearbox) to get the lever as close to a right-angle with the push-rod as possible, without letting anything foul the filler cap when 'relaxed' (otherwise the clutch will slip).
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)