Does anyone know the length of standard rear shocks center line to center line of top and bottom mounting bolts?  What is the longest people have done?  Any in excess of 325mm?

My car recommends 32-36 psi front and 34-41 rear. This gives quite wooly handling and wears the outer edges of the front tyres. I run 42 all round.

I bought a decent 0-50 psi gauge 

I'm actually impressed with how stable it is at speed, although I have yet to test in the wet. I took the tyre pressures from the sticker on the bike, but of course that's for a different construction. I'll give your recommendation a try, thank you.

You should increase to 32 front and 35 rear. It will be much more stable over 50 and in the wet.

I've put a few hundred miles on the Conti Classic Attacks now, albeit only dry miles to date. 110/90 R18 on the rear Snowflake fits fine as you say. And actually easier to get in/out past the rear brake than the 20+ year old 4.00x18 that it replaced.

I've got tyre pressures set to 30psi front and rear, with rear springs on the centre setting. I've nothing to compare it with as I bought the bike as a non-runner (stood 20 years, neglected before that) project, but I'm pleasantly surprised at how agile it feels. It really is good fun to ride. How much of that is native to the R65, and how much is down to the tyres, I have no idea. But either way it brings a smile to my face every time I ride it, and it encourages people to stop and chat about their classic bikes as well.