All well and good keeping them, but the shelf life of magnetic tape is only about 10 years. Good storage and not played to often, or in a bad player, can prolong the life but tapes from the 70s are not going to be a good bet in a new machine. Really bad storage, ie in the Garage or similar, results in a horrid mould growing inside the cassette. If you start to use them again you will need to keep a pencil handy, and cotton buds and pinch wheel cleaner. I can still remember how to repair the cassettes with a snapped tape using a razor and Sellotape, and video cassettes.