Just slides out may need some heat.

make a tool, make a plate with a hole for the shaft. then a spacer to the nut threads now use the nut to pull the shaft. add spacers under the nut as needed to pull the assembly out.

914 310 7086

I had my 1963 Mercury 20hp gearcase apart & it had no threads. My KG7 cover plate had threads but it was so stuck I used a   dremel grinder to cut the cover off & bought a new cover. I later made a puller for my KF7 using a piece from a old trailer hitch with a propshaft hole with two smaller holes for the dowels in the cover. I used the washers & propshaft nut to hold the puller from bouncing out to get it moving it was so stuck. Also used a thread chaser on the gearcase threads to sstraighten the inside of the housing/ Those pieces have been underwater for a very long time plus others tried to get apart thinking they were right handed threads when they were lefthanded threads or righty tighty only they were righty loosely. Take a straight edge & scrape the inside of the gearcase to remove any crud off the inside diameter & rinse with gasoline. If your exhaust spokes aare cracked don’t be so sure until it’s out to clean. I think you are maybe seeing casting marks.