Using standard shocks you need to use the spacer to lift it off the spring hat, I can't comment on fitting them to Spax shocks. But if Compbrake designed it to use the spacer, they did it for a reason. Give them a call, I've already put up their details.
The top nut IIRC is 27mm, but I had to grind the sides of the socket down to get it inside the top mount once it was all on the car as it was easier to tighten when it was bolted to the hub. If you can, tighten the top nut when the shock is still off the car, just get someone to hold it nice and tight.
The rubber 'o'ring sits inside the chassic turret to stop metal on metal contact against the top mount. It should create a sandwich of the 'o'ring and turret between the 2 washers.