Use the throttle cable if you are going 4 stroke. I have 3  FS installs, and all use the cable setup without any special mods needed to make them work.  The key is to make sure you have enough play at the firewall end to allow for the up/down movement of the cable as the throttle arm moves. What I do is put another 1/4" piece of plywood behind the FW where the throttle cable runs through, and slot the exit so it doesn't bind, yet still gives enough meat to secure the cable tube.  I also use the screw type post on the arm as seen in the pic above, just make sure the thing is secured and wont come loose and use thread lock on the screw.