Yes, plug the hole and redo the drill/tap job, but first enlarge it to about a 5/8" thread and install a big plug into the hole. Loctite the plug into it with the strongest stuff you can find, and let it cure thoroughly before doing anything else.
Now, when you redo the drill/tap for the adapter plate, you're not overlapping the repair, you're tapping into a solid piece of steel. It makes a stronger bolt hole than you could get by directly threading the cast iron.
Jerry
