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.
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Can you please tell me how low and high temperature, humidity, pressure, sun and other atmosphere factors affect balloons inflated with air and balloons
I need to send balloons out of the United States for a sweet 16 celebration in another country where they don't have access to helium... What kind of balloons