Do new american standard toilet tanks fit older (1975) toilet? Tank looks the same.

No... you have to replace the entire toilet. They don't cost that much. Besides... where are you going to find a 5 gallon replacement tank from? And the newer smaller ones are not going to work with a older lower section that was desinged to require much more water to actually flush.

Meaning... if you manage to make a new small capacity tank fit and not leak... the first time you take a dump... you might find its not going to flush.