None of the info relating to the MkIII type kits seems to specify setting the clearance with a feeler gauge, but if you wish to do so then I suppose using a version of the earlier adjustment procedure will do.
I would try this:
First, the weight of the bike should be supported by the lower frame rails using a jack, blocks of wood or similar, so that the main stand is not touching the floor if it is a later model that has the main stand fitted to the cradle.
Checking front Iso. unit:
Slacken the central Isolastic bolt locking nut, pull the inner part if the L/H Iso. gaiter away from the Iso unit, lever the engine towards the right side and hold it there.
Then insert the feelers to measure between parts [23] and [24] on the left hand side (as shown on the parts book drawing link below)
http://www.bsa-regal.co.uk/norton/parts ... &Plate=015
The front Iso adjuster should be on the right hand side of the front Isolastic unit.
Check/adjust Iso. clearance then re-tighten the Isolastic bolt/nut.
Check rear Iso.
Slacken either Iso. stud nut. Push the rear wheel to the left and hold it there whilst checking, use the same procedure as for the front Iso. but this time check clearance on the right-hand side, the adjuster being on the left side of the rear Iso.
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Update
Backing off the *original* MkIII Iso. unit adjusters by a fifth of a turn sets the correct clearance, however the *conversion kit* adjuster setting could be different?
The Norvil recommendation for their own kit is back off the adjuster by half a turn.
http://www.norvilmotorcycle.co.uk/techtalk04.htm