What modifications have to be done to change from a #14 head to a #22head on a mercedes 300sdl. Why are they # this way?

The heads are numbered in order of revision. This includes both turbo and non-turbo engines so there are jumps in the numbering for your engine. The most common are 14, 17, 20, and 22 for turbo 603 engines.

The 14 head uses different injectors; later heads use angled injectors (I forget the details). The screws at the front are longer in the later heads (where the timing chain goes).