The easiest way to do this would be to span a fishing line between the last link-o-loons on both sides of the arch, and then tie the megaloon letters to the fishing line. The megaloons need to be helium filled to make them float. I believe they would hold up the link-o-loons, so you could fill those with air.
Here is a good resource with info and instructions around using link-o-loon balloons.
