I have a Durst M605 but with the colour head. I use a Nikon 50 mm lens on a Siriotub and have no problem getting large images. You say you cannot get the lens close enough to the negative holder. I may be wrong but I thought that as the image got bigger you move the lens away from the negative to achieve focus. Have you tried racking the lens down?
Originally Posted by seadd
BTW you can project onto the wall with the M605 by loosening the knob on the LHS and rotating the head around the supporting spigot. There is also an extension column available to extend the column height.
Hope this helps