This is a progress report of the following:
The FD 300mm f/2.8 is still being delivered by Hong Kong Post, way slower than anticipated and it's abnormal. Other parts arrived.
The first one is the most critical, ordered new. It's a M42-EOS adapter with chip which allows focus detection. The FD 300mm f/2.8 should have 77mm thread upon removing the rear aperture control part, so a 42-58mm adapter was sandwiched between the M42-EOS and a 58-77mm stepping ring. It's rather compact and strong enough as shown below:
However, as expected, the amount of 77mm thread is a bit little since it's originally designed for filter, but now it's going to hold a lens-camera combination.
In order to create the proper flange distance, a 3-piece extension tube will be used to connect to a camera. The FD 300mm f/2.8 will then become an astrograph or for regular imaging. In case the combination could not meet the flange distance, an adjustable M42 tube should be ordered.
To be used in visual mode, the extension tubes would be removed, and instead, a 1.25" barrel adapter will be used, plus a diagonal which should probably make up the optical path length to allow infinity focus.
Of course, the combination above might be a bit optimistic, and might require some more fine tuning, like adding extensions.
You know, I am getting excited. A poor man's portable astrograph and wide field 4" refractor all in one! Even an expensive new EF 300mm f/2.8 will not be able to serve in that visual configuration without a 2x barlows as a relay lens! Best of both worlds.