How long do electrolytic capacitors last in seldom-used equipment?
If you're already in there messing with it all over the place, I'd replace all the big ones for sure. All of them if you like. With replacements from a top-tier manufacturer. 

Otherwise, no need to break something that isn't broken (broken would be excessive hum or possibly intermittent oscillation). 

Lifetime is related to time and temperature mostly, and temperature goes up when in use, so the lifetime is reduced. 40 years is not uncommon for good capacitors kept cool, but it's also possible some have excessive ESR. It's usually not a catastrophic failure, just an increase in ESR (equivalent series resistance) as the electrolyte dries out. At some point the circuit doesn't work well enough. In the case of your power supply that would be excessive ripple and probably some resulting hum. If there is regulation in the power supply the onset of hum might be more sudden than gradual. 

As a rule of thumb life is halved for every 10°C temperature rise, so it's usually good to buy 105°C-rated capacitors rather than 85°C, all other things being equal. The lifetime ratings at full temperature are very short (thousands of hours only). Higher voltage rating than the original is also better. Since capacitors have gotten smaller over time for the same ratings you can definitely substitute a really good part that fits in the same space as the original.