If the leach field, not the tank, has not been replaced since 1964 my $$$ is on the leach field needing replacement. That would make the field 52 years old and that is the upper limit for one to last.
If after 1.5 weeks you are having the same symptoms then chances are you need a new one.
Are there any trees or bushes between the house and tank or within 75' left or right of the soil pipe going to the tank??