See p 9 of the instruction manual:
http://www.jollinger.com/photo/cam-coll/manuals/enlargers/omega/C760 Modular Enlarger System.pdf
Paragraph 6 under Lens Mounting:
"...Due to the varied back focal distances of 50mm lenses, some will require the use of the recessed lens mount in place of the flat lensmount"
So that's one thing to check. If your enlarger has only the flat lens mount and didn't come with the recessed/extended lens mount that was also included in the original box, you might try KHB Photografix to see if they can still supply it:
http://www.khbphotografix.com/omega/Enlargers/C760.htm