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Nice, congratulations!
It has the original 1968 red band Merc? One of my favorite year Mercs. BUT, you have some work to do before you try to start that engine. The main issue with older Mercs is corroded harness wiring and carbon steel shift shaft and driveshaft which typically corroded in the seal area. The water pump impeller is toast so you need to drop the gearcase and replace it otherwise the impeller pieces will break off and try to migrate up into the powerhead cooling passages causing a host of cooling problems. While the gearcase is off you can see the level of corrosion on the shift shaft and driveshaft. Post some pictures of what you have.