Horizontal tanks do not need re-certified, but a system that old should be throughly checked out by someone. (expert) You may still run into problems with places refusing to fill your tank, because it doesn't have an OPD valve, even though it's legal.
Also, and this goes for everyone, check all the screws holding the ground wires on the bus bar, and check the wires going to the breakers. These tend to vibrate loose, and can short frying a breaker, or cause a fire. These should be checked at least once a year.
