Water heater straps too short. Replace straps? Get extension?
If you haven't got access to replace the strap, I'd just replace the bolt with a piece of threaded rod and a couple of washers/nuts.
They won't lie flat to the brackets, but since this is just for stability, they don't need to go tight so this shouldn't matter.
The above is obviously assuming that you can run a straight line between the holes on the two brackets, but it does look like this is possible from the picture.