I like using my Nikon 500mm F4 P on my Nikon film cameras where I can get away with the slower film speeds (compared to a DSLR anyway) but then I bought a Pentax K1 on impulse and only had my manual legacy lenses to use and didn't really want to buy modern glass for it.
I figured I could have my cake and eat it by changing the mount on the Nikon TC-301 2x teleconverter made specifically for these long lenses. They're meant to be pretty good so I wouldn't lose a lot of IQ but the lens itself would retain its Nikon mount.
The Leitax adaptor did need modifying to clear the second lever on the Nikon lenses, which involved filing another notch but it wasn't a huge deal. The only problem is I can't move the aperture lever on the lens past F4. It might need some further tinkering but although it'd be nice I don't see myself going past F4 with this combo as the shutter speed will tank...
I finally got to use this in anger today as it was a particularly sunny day so I took it to Cake Abbey
The camera with the 1000mm F8 lens complete with 3D printed hood!
A bird, taken with the camera sat on a bench, angle adjusted using twigs under the tripod foot
Cropped:
My favourite subjects... sheep! These were taken handheld as there wasn't anything to rest the camera on. My tripod isn't beefy enough for it.
Cropped:
And another sheep:
The results were way better than expected. I don't expect to use this setup very much but it was fun!