Lowering 1966 N15CS

New boots would be the answer?

Sorry, one too many sherbets tonight!

I would suggest that changing the rear shocks are your best option as the CS had longer shocks than the other G/N15 range, having seen inside a late'65 N15CS forks the dampners are special [2 way damped? ] and different to normal Norton's so I suggest don't muck with them.

If the rear shocks are shortened or lengthened by more then a small amount then the run of the rear chain will change and possibly touch the swinging arm. 

At the front end shorter stanchionscan be fitted but this can bring the standard mudguard nearer to the front downtubes and cause issues when braking hard.

How about obtaining a set of smaller diameter wheels (17") and not riding over speed humps too fast?

N15CS runs a 325x19 front ad 400x18 rear.

The 1966 N15CS, has the later standard Roadholder internals so it would have the internal springs with the standard type dampers. These internal dampers were an inch longer than those used in the Atlas and 650ss models. The only models of the N15CS, G15CS that used the G80CS type front fork internal were used from 1963 to 1965. The rear Girling units are about an inch longer on the N15CS so you could fit the Matchless G15 Mk2 or the Matchless G15CSR rear Girling type units. The 1965 G15CSR bikes used 18 inch wheel front and rear wheels. I have seen N15CS and G15CS bikes use the Commando length fork stantions, but the handling may be a tad off. The stantions on the N15CS are 25 inches long or 24 7/8 inches long. 

I would not be shortening just the rear shocks if the bike has already been modified to be taller at the front, you are just going to amplify any adverse effects. I would be lowering both front and rear.