You're going to need a long box. I would talk to some local body shops and see if you can get a box that a pick up box side comes in. That should be big enough.
There was 1 offer and one acceptance post deleted. This is not a parts wanted or sale thread and you guys should not treat it as so. If we see offers to sell orbuy in general or informational threads we delete them. Send the guy a PM but don't sell in a non sale thread.
How much bigger? say 1" to 1 1/4" ? I wouldn't mess with a die grinder or drum sander. They are way too hard to control in a hole like that. A step bit would work well.
Start a parts wanted thread here. I bet someone will have one. Shipping is the worst part. Swap meets are the best. Do not offer parts for sale here. That's what wanted threads are for.
ANother thing you need to think about with powder coating is that it has to go in a oven to cure. A 8 foot piece of chrome might be more than a challenge for most coaters.