You cannot let a Silver Shadow sit. That's the worst thing you could do, the car is only a daily driver, the more you drive it, the better the car will work and perform.
To be honest, installing seat belts isn't the biggest deal with any Rolls Royce. These cars are built like tanks, and if you or any passenger dies in an accident behind the wheel, then that accident was almost unsurvivable period. These cars have superior build quality and materials compared to most of the stuff being turned out today, so you really don't need to worry too much about safety in a Rolls. Just enjoy the car the way it was made, because if you begin tampering with the car and changing it up with parts and features that are foreign to it, is it still truly a 100% Rolls Royce?
I have seen some of these at cruise nights and shows, re-powered with domestic drive trains. It will cost as much as the car to convert, but then they have modern reliable drive trains. I just saw one Chevrolet powered at the Hagley 2011 annual show. I have also seen Jag XKE models likewise. I know of one with a Ford 351 in it. Drop a Jasper engine, or better yet a stroker in one. I like stock cars, but if the engine goes, set it aside. I like better tires, disc brake upgrades, cooling without guilt in older cars. I also like modern electric gauge clusters vs the cars with electrical gremlins.
All Australian, and many other, delivery Shadows had seat belts installed by the dealer locally, and there is provision for installing them built into every car. Retractable front seat belts have the retractors under a floor plate next to the front seats.
Rear seats belts however have the retractors mounted on the parcel shelf - these are usually covered with vinyl or leather, and dyed to match the shelf.
All the technical information and advice you will ever need for these cars is available **free** on the Australian RR club website. Just google RROCA, and go to the forum for discussion, or the technical section for copies of everything from the mechanic's workshop manual, to instructions for fixing inevitable minor annoyances and more.
Also on the site is Tee One topics - read and enjoy.
My 1974 Shadow is my first (and last!) car, and I don't regret a penny spent on her.
Two pieces of advice for the novice:
1- Get an inspection done by an RR expert on several cars before deciding to buy. Every $100 you spend before you find a good one will save you $1000 afterwards.
2- Drive it as often as you can afford to put fuel in it. Low mileage is not necessarily a good thing on these engines. Repeat after me: Drive, Drive, Drive.