If you know the shaft needs to come out why risk damage to the gear box or gear box flange by trying to press the coupling off the shaft. It is far easier and far less time consuming to simply cut the shaft out. This took all of about 45 seconds to free the 1" shaft from the flange.
I will type this loud and clear so it hopefully makes sense: NEVER, EVER, EVER, EVER use a "Slide Hammer" to remove a shaft from a coupling if the coupling is attached to the gear box! If you want to throw 3k out the window feel free to use a slide hammer, if not, use PROPER procedures for removing the shaft from the coupling such as a coupling press tool.
Only hacks, that don't care about your boat or gear box, use slide hammers to remove prop shafts from the couplings when the couplings are attached to the gear.. Yes it's quick & dirty, and the damage to your gear box can largely goes unseen, for a period of time, but DO NOT be fooled by yard monkeys with slide hammers and do not allow a yard to use one on your boat. Would you go to a dentist who used a Milwuakee Sawzall???? About the same level of stupidity and it is the WRONG TOOL FOR THE JOB.
A slide hammer is essentially a long piece of metal pipe that threads onto your shaft where the prop nuts go. It is about 4 feet long and has a heavy metal weight on it. The weight is slid up the bar to the prop shaft and then thrown or "slid" down the bar until it hits the end and SLAMS to a sudden stop. The idea is it breaks the coupling from the shaft when the weight comes to the suddent and abrupt stop. The reality is it destroys gear boxes in the process.
This destruction is not readily apparent so yard monkeys assume they get away with it and it "works". Yes it works, it works to DESTROY gear boxes. If your boat yard tells you "we do it all the time" please do yourself a favor and find a new yard.
Slide hammers can cause brinneling of the bearings or races in the gear box. This means the shock loads imparted on the static bearings, by the "slide hammer", create FLAT SPOTS in the races or bearings. The gear may work for some time after the slide hammer event but eventually the damage rears its ugly head.
About ten years ago I was at a yard when I overheard the yard boys slamming & slamming & slamming a slide hammer to free a shaft from a coupling. All of a sudden I heard one last SLAM, then a clunk and the sounds of metal bits on fiberglass, then "OH $HIT!"...... You guessed it they hit it so hard they blew the case of the gear box apart and destroyed it. Literally cracked the iron gear box wide open. The shaft, after all the beating that finally destroyed the gear box, was still firmly embedded in the coupling.