Not that hard to adjust the Merc shifter. Here's my procedure:
1) Make sure the upper control cable which comes from the control box is adjusted to a distance of 7 & 1/4ths inches from anchor pin to anchor pin with the control in neutral. Adjust the barrel on the cable if necessary.
2) With the control in forward adjust the barrel on the lower shift cable to fit the cable between the mounting studs with the cable collapsed and the prop locked in gear counter clockwise.
3) With the control in reverse adjust the front cable mounting stud in the slot to fit the cable between the mounting studs with the cable fully extended and with the prop locked in gear clockwise.
4) Repeat steps 2 & 3 to verify their settings.
5) Reinstall cable mounting washers, nuts, and cotter pin.
When tightening the nuts that hold the cables on their anchors do not make them so tight that they can't pivot on the studs. Leave a little play for the washers to spin freely between the nut and cable ends.