The spline on the MS3 reverse gear
input shaft engages the spline on the spring damper plate about 1" to 1.25 ".
If you can move the prop shaft aft that much then its an easy job. If you cant move the shaft that much, you can still get the gear off but it takes a little more muscle.
Here is the way to do it:
It might be helpful to remove the exhaust
hose from the exhaust elbow
to obtain better working room, depends on your installation
Close the thru hull
valve and disconnect the water cooling
hoses from the gear box. Remove the shift cable. Break loose and remove the four bolts holding the prop shaft coupling to the output flange of the gear and slide the shaft coupling aft at least enough to get the coupling pilot disengaged from the pilot recess on the gearbox
Loosen and remove the six bolts holding the MS3 to the flywheel cover plate. Wiggle the gearbox
housing a little to get it loosened from the pilot in the flywheel cover plate. Lift
up on the gear housing a little so that its loose in the cover plate and rotate the entire housing 90 degrees to the left or right, which ever is easier. Because its a drop center gear, this will rotate the gearbox output flange up and away from the prop shaft flange and allow you to slide the gear box aft and disengage the spline from the spring plate. Then you can set the gearbox aside and remove the flywheel cover plate and expose the flywheel. Reverse the procedure on the way back together.