I did that repair on a 301 Turbo block. I placed a grade 8 bolt in the threaded hole with the head on the engine side, the shaft sticking towards the trans side. Then I welded it in place. Welding cast iron is tricky. If you can or know a buddy that can do it, then make the repair. What I do is use an acetylene torch to melt and flow the iron where I need it. The block is a huge heat sink so I use a very large tip to get the area I need melted. When the metal turns yellow/white it can flow. I use the torch pressure to push the molten metal around where I need it. You can even use a cutting tip to goose the molten metal around. You are not trying to weld the steel bolt to the cast iron block, but rather re-forming the iron around the bolt to hold it in place. You can use a broken chunk of iron as filler. If you are not comfortable with that an alternative is to braze it in place. Cast iron does braze easy with a flux coated brass rod. You still need to get it very hot but brass melts at a much lower temperature than cast iron. As bigchief said one bolt doesn't matter all that much but I prefer to have at least something there to secure the transmission.