[AZKen]

How long a gasket will last is a complicated question. I never buy NOS carb kits or anything with diaphragms or rubber or cork, etc. If the gaskets you will be saving are old technology material or rubber or cork they will not only shrink dimensionally but will loose compression (durometer). If the gaskets are newer materials, and as you say, kept in temp control, flat and dry, they will last a long time. Buying gaskets and storing for many years for a 305 will have a risk. But what else can you do? I agree. The only plan B would be to make templates from them before they shrink, save the templates and have some gaskets made when ready to rebuild. The only gasket that could cause damage that I can think of is the head gasket. The others will just leak. There are many places that you can use sealer instead of a gasket now-a-days.
If gasket sets for V6 are not being made and the gaskets everyone is buying are old stock, they are being stored someplace, may have been stored for a long time and unknown if stored properly. They will shrink, outgas, dry out, thermal cycle (heat & chill) and ozone will damage. All depending on what material they are made from.