Sure, the 300mm Fujinon-C is a great alternative: small, multi coated and a great performer. But the Nikkor 300mm M f/9 is also small, multi coated and a great performer. Just buy a step-up or step-down ring to accomodate your needs. Both are outstanding lenses.
I also own one of the Nikkors and think it is a great lens. I don't know whether you've used one, but if you haven't --and you'd like to-- I wouldn't mind lending you mine. I'm in town, so it wouldn't be a problem.
Midwest has both a Nikkor 300M and a 300 Fuji-C used. Why look further? The Fuji is probably my old lens (had too many in 300mm which isn't my favorite focal length). If it is, it's mint and has the box and papers too.