Both? Isn't one of your ST's a 1300 which has timing chains.
I really don't know the correct answer to your question and I have no idea what they are made of so I wouldn't even know what to look up.
I would use the same logic as storing V-Belts and tires, which is to keep them well away from any possible exposure to sunlight and ozone, ozone exposure being why tires should be stored in bags, ones that are not clear, even if they are kept inside a building. Keeping them within the same temperature and humidity ranges as what keeps humans happy is a good idea as well if that is an option.
Being a belt I would also make sure that it is stored in a relaxed position, i.e. Not folded or twisted with a tight radius bend in it. V-Belt manufacturers recommend against storing belts by hanging them. If they are to be hung they they should be hung from something with a radius at least as big as the minimum radius sheave that it is designed to be used on. They actually don't recommend hanging them at all. The preferred storage method is lying flat in a relaxed position.
