I have done this on a 1984 70hp johnson.
Here is a thread where I helped someone else.
https://www.ifish.net/board/showthread.php?p=8083178
This guy talks about it too...
https://www.leeroysramblings.com/converting_to_tiller.htm
This is the piece that I modified to get the carbs to open and close the same amount with the shorter throw. The remote controller throttle cable traveled something like 2.75 inches and the tiller throttle cable traveled 1.5...those numbers are close. I marked the location of the carbs at idle, start, and wide open throttle (both where the carbs are and where the roller on the cam is. Then I started by removing the cam and tracing it on a piece of 1/4 plywood. and then reinstall OEM cam and I connected the tiller handle. and moved the throttle to all three marks. and measured the distance that I would have to add to the cam to get the carbs to the position in relation to the marks on handle for start and WOT and added that much material to the plywood template cam in those locations. Cut our the wood cam and test. If the roller travels smoothly on the cam and it puts the carbs in the correct positions then you are really to water test. I used washers and my plywood cam for first test. When happy with the performance create a aluminum version of your plywood cam. NOTE* you will notice the level that connects to the throttle cable all so advances timing don't worry too much about this as that timing carrier travels its full distance early in the movement of the throttle travel. I was careful to out change the relation of the time and position of the carb linkage until timing was fully advanced. I have run the motor for a year now and love it as a tiller. Probably been 6 years now...no issue. It made the boat MAY more fishable.