A gasoline's octane rating basically indicates the temperature at which
it will NOT preignite. Preignition is a fuel's tendency to self-ignite
before the spark occurs; such preignition is caused by the engine's
compression ratio, glowing carbon deposits and any hot spots in the
combustion chamber. Anyway, one doesn't want preignition or "pinging" to
occur because the fuel explodes before the piston reaches the very top
of its stroke so the engine works against itself so to speak.
"Pinging" is more likely to occur in hot weather than in cooler weather
so if your 450 doesn't "ping" now the fuel is OK to use. You've already
experimented by using mid grade fuel, you could try a few gallons of
regular in a relatively empty tank so if it pings there's tank capacity
to add some high octane fuel to eliminate the pinging - end of
These '70s gas engines were quite detuned to achieve emissions goals.
Detuning included lower compression ratios than used in Europe, USA
versions' power suffered but that was the technology available then. So,
the engine may be well able to run on regular fuel, but "pinging" is the
warning of its limits.