I am not sure they would be rebuildable and if they have been run a long time low on oil the bore may be too worn
Would it not be easier to get AM shocks?
Most of the bikes had little use so internal wear should be minimal. I plan to see whats up with mine and then go from there. You can fiddle with rebound rates by changing oil viscosity then moving to reshimming the valve stack if you want more, bearing in mind the spring preload of the stock spring etc. Some aftermarket shocks will be great but wont look period as those of the later 400s do.
As the bodies are aluminum it's safe to assume the top bushing and seal is a screw on item, It needs some investigation.