These things happen all the time. GM brand "C" has a hole in the front of the block that was once a motor mount. It's also for holding the the fuel pump push rod when changing a fuel pump. It will leak oil if you don't put in a short 3'8" cap screw. Think of all the head bolts on various engines that go into the water jackets.
When I'm working on an engine that I don't do all the time I keep a phillips screwdriver and stick it in every hole before I attach something. I'm suprised how many go into the water jackets or into the crankcase.
Every mechanic has his favorite sealer and mine is Permatex, or other brand, #2. lt's great for water, fuel and oil and one I learned from my dad as a teen ager and I'm now 67...........Good Luck
56 Vic, B'Ville 200 MPH Club Member, So Cal.