IIRC there is not much space for extra washers (bolt should not be long enough). Also, if you use a split lock washer you should use a regular washer to keep the split lock washer from gigging in casting. I do not think there is space for both.
Do not worry about thermal expansion - the length of the bolt and the temperature difference that this thing encounters is not worthwhile worrying about.
Tighten the SS nut to a point you would have tighten the nylon nut and you are good.
Check the holes - in the casting and in the pylon. In my case the ones in the rear pylons and the matching castings turned oval. I decided to file them round to the next bolt size to reduce movement. I also used this opportunity to shim the pylons / castings.