Very good to hear you're pulling the engine; it's becoming apparent that the flange is very well stuck on the shaft.
Plus, once you get the flanges separated there is virtually no way to cut the flange off the shaft because they can't be separated far enough to get a grinding disc in there.
For a little more specificity on separating the flanges, the wood chisel method has had a near 100 percent success rate for me (near
because I can't remember
it not working), and is quick to boot, but if you can't get it to work, some of the other suggestions might help, but one to stay away from is driving the bolts out with a punch. Doing this risks damaging the races, balls or rollers on the output shaft bearings, and should be considered only as a last resort.
As for cutting the flange off after you've removed the assembly from the boat
, my favorite tool is a 1/16" thick metal cutting disc on a 4 1/2" grinder, though a 7"x 1/16" disc can often be mounted on a 10 inch table saw for better control. A metal blade in a sawzall will also work but may be a bit more tedious, depending on the quality of your blade.
For the actual cutting, just a single
slice parallel to the shaft is probably all you'll need; if you cut parallel with the key you'll have less flange to cut, and avoid scarring the shaft (which, if it happens, isn't the end of the world).
And, of course, cast iron cuts very easily with a hacksaw...