Hi Blaine, I had almost the exact same problem last winter - my cutlass bearing was worn out and the shaft has to be pulled to to replace it. I hate to break it to you, but it is an absolute pain in the a** job, but it can be done. It took us three or four weeks of working on it every two days or so to get it off.
Step 1 - Prep - First thing I would do is to get as much of the support structure for the cockpit and other stuff out of the way, I smashed my hands and wrist on it a couple of times when the socket wrench slipped, and with the amount of force you have to use to get this thing to budge, it HURT.
Step 2 - Penetrating Oil Soak - Next I would do as other have suggested and soak the ever-loving piss out of it with Kroil (which I think is what Russ is referring too, works wonders) or SeaFoam penetrating oil, but I would start at least a week, if not several weeks in advance. Put an oil adsorbent pad underneath, soak thoroughly, rotate the next day, soak, repeat ad nauseum for as long as you can stand. Penetrating oil works, but with something as seized up as these bolts are, it can take a while. It felt like whoever put the prop-shaft back on the coupler used a hydraulic press on ours. You can also heat it gently with a hot air gun at the same time to help the oil soak in.
Step 3 - Removing the coupler bolts - I had a couple spare lifeline stanchions, and when cut down they make perfect extensions for a 1/2 inch socket wrench, you are going to need the extra leverage. If you don't have one, buy a spring-loaded automatic center punch like this one (
https://www.homedepot.com/p/Husky-6-in-Automatic-Spring-Loaded-Center-Hole-Punch-70079H/302880581), file down the head so it is round or flat, and whack the bolt head a few times with it. The short, sharp impact helps to free up the bolt and it won't munge up the threads like hitting it with a hammer could. Grab your socket wrench with *six*-sided sockets (you need as much bearing surface as possible for this job) and go to town. You will need a second person to help with another wrench to brace the bolt closer to the engine with the largest Yanmar wrench against the engine pan to stop the coupler from spinning. It can be done with one person, but it is tedious and the 2nd wrench will slip a lot, I held it gently in place while apply pressure and eventually it will be forced into place and you can pull the wrench with both hands. Once you have all four bolts free the prop-shaft and the coupler should come free of the transmission at this point, if it doesn't, a few good whacks with a hammer should convince it to let go.
Step 4 - Pulling the coupler - Now comes the fun part, you need to build a press inside the engine compartment. Grab four grade 8 bolts and nuts that are the same as the coupler bolts but about an inch longer (yes, we had to make *a lot* of trips to the hardware store - I would buy many extra nuts, they get beat up) and a socket just smaller than the prop shaft. Insert the socket so that it pushes against the back of the tranmission and the prop shaft inside of the coupler but not the coupler itself, I used a pair of wide needle nose pliers to hold it in place while I placed the coupler/prop-shaft combo up against it. Now thread on your extra long bolts a tighten the whole contraption down, a second person comes in handy to hold the socket in place while you get it tight enough to stay still. Next, work your way around the coupler bracing the opposite bolt against the engine pan using the large Yanmar engine wrench - the pan can take the force, it is extra reinforced in the stern. Tighten the bolt so that it pushes the wrench holding the opposite bolt *down* into the engine pan, there is almost no way to hold it against the forces you need to apply to break it loose. Work your way around the coupler swapping in longer bolts and sockets as needed, and after much sweating, bleeding, cursing, wailing, and gnashing of teeth the coupler should eventually separate from the prop shaft. Heating the coupler helped as well, but you need to get it very hot, likely smoking hot to help, so wear gloves when warm.
Et voila - your prop-shaft should now be free, in four "easy" steps. I would recommend buying a split coupler and coating it with POR-15 rust-preventative paint so this doesn't have to happen again.
