Sorry for not updating this thread.
Yes, I got the paperwork approved, and the shotgun should be returning from Wilson Combat any day. I decided to go with a 12.5" barrel.
As for C&R status, there are two items here. According to Federal law, if a firearm is 50 years old or older it is a C&R. Now if it is a military firearm and is imported, it must be in the original configuration in order to be imported. That requirement is only for Military firearms that are imported.
Michigan law says that short barreled rifles and shotguns are illegal unless the Secretary of the Treasury, (ATF) considers the firearm to be a curio, relic, antique or collectors items not likely to be used in a crime. So the law basically says that if the firearm is considered a C&R, you can put a short barrel on it. They leave the determination up to ATF. ATF says in the case of my shotgun, yes it is a C&R, since MI only cares that it is a C&R to begin with, it is approved.
I have three items approved right now. The first one was approved over 6 months ago. A Sistema pistol that I added a stock, a Canadian centennial Ruger 10/22 now with a 6 1/2 inch barrel, and a 1952 Remington 870 with a 12.5" barrel. I have the Michigan "safety inspection" green cards required for pistols and longarms under 30" in length. So the state knows about them and they didn't have a problem with it. (Well they did make me bring the 10/22 back in to verify the serial number since it is only 3 digits, they thought some digits were missing on the form.) I made sure to list the category as short barreled rifle or shotgun and had the barrel length listed.
I have a Marlin 336 that is next. So far I am the first person in Michigan to find this route toward ownership of these NFA firearms in MI.