Putting on Mirror Extension Kit 91907-87, noticed that the spacers that are supposed to slide over the mirror stem don't fit, do you have to do some sanding on mirror stems, or spacers?

 

No sanding they should slide right on it is a simple job. I have done it on every touring bike I have owned.
Read number two againmake sure you have them the right way
Installation
1. Remove mirror from handlebar. Retain hardware and
note sequence for reassembly.
2. Slide spacer (chamfered end toward mirror) onto
threaded end of mirror stem.
3. Apply Loctite to threads on mirror stem and thread into
mirror extension.
4. Mount mirror to handlebar. Do not include original
equipment spacer from top of mount. Add flat, then
lockwasher to bottom. Secure assembly with acorn nut.
5. Repeat Steps 1 through 4 for opposite side.