You are most likely running too lean at idle. As in, not enough fuel at idle and then shifting into gear puts load on the motor (with not enough fuel..) ... It will die every time.
Search iboats for "Tower of Power idle adjust" and you'll find some good reading.
To get the idle right, you need to be at the dock or in test tank because adjustments are made with motor in gear.
At the dock, you'll be adjusting the idle screws on the 3 carbs. Turn the screw right leans the idle mix (less fuel). Turning left richens the mix (more fuel). You'll be turning left in small increments 1/8 turn at a time.
Again, search for forums as others have done great write ups on the procedure.
I have a 1975 motor and spent a few hours learning how to get the idle right. I can idle all day at 900RPM and then shift in/out of gear with no problem. It took some time to get the idle adjust right (each carb had its own adjustment), but working great now!.
