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I had an exhaust leak so I took the exhaust flanges off the head to change the gaskets, but it turned out that the leak was because one of the threads on the head was helicoiled, and the helicoil had completely stipped out the hole. The hole is now huge.

There are another 2 holes that aren't used, and it looks like the flange should bolt up to them pretty easy if it is welded on at a different angle.

Does this seem like a good idea or is there any reason why it wouldn't work?

Here is a picture of the holes I am reffering to:

If you can cut and re-weld them using the other holes, go for it. That's what the exhaust shop did when I got a new exhaust made.

I have see flat plate welded/drilled under the other holes, then all four used.
Theoretically, much stronger.

Thanks, I'll do that

The curved sides of the flanges were ground straight so the weld would not be drilled through and the holes will drilled oversize to allow for movement.

Here is the fix. It has worked well, there are no leaks there now.

There are 4 stud holes because the heads can be swapped to either side of the block, so you need to e able to swap the stud location to suit the exhaust headers. The later (87 onwards) EA81 engines used the second set to hold the EGR blocks.