Also available in a Monogram metal body version. Just keep checking eBay from time to time and you may find an acceptable price with shipping. I don't know if it's the same as the Testors / Bburago metal body kit.
I've seen plenty of leather, fabric and other material gun cases for a single, fully assembled shotgun, but not like the one you're making. But I guess It would be custom-made anyway. This project looks fabulous.
You'll notice in the link to a built kit (one of the best I've seen) that the hood doesn't fit properly and needs attention. Again, you'll want to find some larger tires. As mentioned, lots of detailing and accurizing needed. The windshield frame needs attention, too. The Lindbergh box is very misleading since it shows a 1:1 car, but at least it provides reference.
I have Grandt Line delrin chain, the smallest I could find, which is still too wide and thick for the brake cable for my 1/16 Bugatti, and would be too big for a 1/24-25 bicycle. That Top Studio motorcycle chain looks possible for my needs, but I'd sure have to check actual size before I spent the money.
Yes, the Auburn wheels and tires are too small in the Lindbergh kit; they were more accurate in the original Pyro version, but they were 2-part plastic wheel/tire molded together. However, the Auburn is 1/25, so you'd need to check compatibility with the Cadillac.