I converted a B13 harness to fit in my B14 and now its going into my P10. I dont like step downs unless they are made with new connectors, wires, and the pins are crimped with the proper crimper.
The only thing that has to change is the car side connectors, for this you will need the B14 side and solder them onto the B13 harness. Depending on what sensors, maf, and dizzy you are using you might have to swap a few more plugs over. While doing this you can also clean up the harness and bit and make it fit a little better.