I think you need a different lens.
This link will initially seem to have nothing to do with your problem, but bear with me: http://www.khbphotografix.com/omega/LensMountD5.htm
The reason this chart may help you is that an Omega D5/D6 only works with certain 50mm lenses because the bellows has to be tightly compressed to work with 35mm film and a 50mm lens. And when the bellows is tightly compressed, a lens that protrudes through the lens mount fouls the mounting system.
If you look through the chart, you will see that very few 50mm lenses work with the shallow 421-105 lens mount. Those 50mm lenses are more likely to work with your enlarger.