Please DO NOT over tighten the draw bolts. This boater was very lucky he broke the shaft flange and not the gear box flange. Despite his gear box flange not breaking, it is now severely OUT OF TRUE. The only way to fix this is with a rebuild.
Despite it not breaking it still cost $1750.00 to fix the BENT gear box output flange which required a re-build of the gear box. Remember these flanges are going to be aligned to 0.003" when you put her back together. It does NOT take a lot of force to bent the output flange by 0.003".
If the coupling won't come off you can cut the coupling just over the key way as seen in this photo. Creating the cut at the key way will ensure that you don't damage the shaft. One cut it will relieve the pressure and the coupling will then come off.
If you can't get an angle grinder in there to cut the coupling you may need to cut the shaft. Not all couplings will come off so PLEASE don't ruin your transmission in the process.