rc13, a half hour job to install that Hellwig bar, given no stuck bolts. You should not have to drill/file/cut anything. On ours, the OEM sway bar bolts were rusted into the weld nuts under the axle. One took more time than the whole rest of the job.
Yours is a later E450 than ours, but I'm guessing the bars are the same. If so, think about putting the long front link bolt UP from the bottom instead of down from the top. That way you have less bolt end sticking down to get into things, and there's no space restriction on the top.
Please give us an update on the job, how it goes, and the results it provides.