Advice on archiving old letters

I am a complete amateur in possession of an old strongbox full of letters my grandfather wrote my grandmother between the mid-1930’s when they met and he was a Merchant Marine through WW2, and into the mid-60’s when he would spend long periods at a rest home for veterans. He died young in 1966. I want to save them as they are an important piece of our family’s history and I am worried about them degrading. Any and all suggestions would be appreciated. I wasn’t sure if I should open the letters and store pages and envelope between acid free tissue paper, put into sleeves, leave in their envelopes, or what to do. But I want to get this right so I bow to your expertise. They’ e been in that strong box for a good 60 years, and not sure if that’s a good place for them. Thank you in advance.

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