First line of insurance is a safe, bolted down to the floor. One thing I've learned is that a valuation can be done correctly, with a sound price, and still be successfully shot down by an insurer.
The best way might be to start with the insurer, and get in writing who they will accept and what they are looking for in a valuation.
I gave up buying riders for my own guns (nothing as valuable as what you have) and consider the safe the main insurer.

Best,
Ted