Re: 1964 Merc 650 Powerhead removal?
Have you taken down the lower unit? Sometimes the shaft can be seized to the powerhead. My powerhead was a bugger to get off! I took an xacto saw and carefully cut the gasket between the exhaust housing and the power head in several places. Then I wedged popsicle sticks in the openings. I sprayed PB Blaster in the openings to soften the gaskets. I did this all around the gasket. Next, I got a Painter's pry bar, it has a very thin leading edge, was able to get in the openings an give it a jiggle. Once again ,working all the way around the assembly. Keep adding shims were possible. It's important to come up evenly. Cut away as much gasket as you can see. Also spray more penetrating fluid on the studs. It's tedious, but it might save you form cutting a hole in the mid section.
Ed