Anyone know where to hook up a tachometer on a 73 Johnson v4 115?

I want to see where we are at. Make sure we are not over/under
propped. Also if anyone knows what RPM it should be at WOT would be
great also. What RPM planed out should one get the best Fuel miliage
out of it? I am guessing 3500 rpm. But being as how this is a barge it
never truly planes out I dont guess.

Thanks