Try this, Rob. I think your key here is to use Quark’s import command. Be well.
on open theFiles
repeat with i in theFiles
set filePath to i as text
tell application "QuarkXPress™"
activate
import file i to text box 1 of spread 1 of document 1
end tell
end repeat
end open
Oh, and I just realized, you didn’t even need the “filePath” variable declared in my script. So here it is in true form. 
Of course, this one assumes you already have a Quark doc’ open, as your script did as well.
on open theFiles
repeat with i in theFiles
tell application "QuarkXPress™"
activate
import file i to text box 1 of spread 1 of document 1
end tell
end repeat
end open