As for making a B&W negative from a color slide, it is easy. I have made 4x5 negatives with 100 speed panchromatic film from 35mm Kodachrome slides in the enlarger. If I remember correctly the exposures with my enlarger were around 5 seconds at f/11 or 16. You will need to bracket to get the correct exposure. You need to have a black surface under the film so you do not introduce reflections through the film base.
Somewhat off topic, but which film to make a color internegative to print C prints?