re capping

Electrolytic caps kept fully charged will function perfectly well and not degrade!

Nope. There are multiple mechanism for electrolytic capacitors to degrade. We once replaced 7 million of them.

The only way to properly test caps is to pull them all out and test with a quality tester. A cap test on a VOM is probably not adequate.

ESR increases as electrolytics age and increased ESR is never a good thing.

Electrolytic caps are rated for 1000 to 10000 hours @ maximum conditions. If operated at less than maximums, life can be doubled for every 10°C below max °C. Assuming 105°C caps and a 35°C internal temperature, that's between 125k and 1.2 million hours. Only a small percentage of the caps would be expected to have the upper limit. 1977 was more than a million hours ago, so a 40 year old amp is living on borrowed time.

If the ampzilla has never been recapped since 1977, as a technical type, I would never even turn it on without a full inspection and some critical level recapping

Ditto! 1977 is 40+ years and that's nearly 3 times as long as the recommended de-rated 15 years due to seal deterioration.

The Truth is available from the manufacturer.
Sometimes the Truth is optimistic and sometimes not.
Life is predicted based on science and math, but can vary widely due to variation in sub contractor supplied materials.

We once replaced 7 million 85°C/35V electrolytic caps with 105°C/50V version from another manufacturer. All was well for a while until we found that the replacement would fail if the bung was over the -18v rail. The potential sucked out the electrolyte and started fires once the electrolyte reached the positive rail.