Voltage doesn't climb for no good reason. I have a vintage Super Twin that's supposed to be at 500 but it's at 530 now for the same reason -- climbing wall supply voltages.
I wouldn't let the caps continue to operate beyond their rated voltage. That's just begging for a bad outcome to occur. So I'd do something about it.
Other options that you haven't listed would include:
- external bucking transformer
- voltage divider on the B+
Personally, I'd try to avoid raising the impedance of the supply rail, so I'd bring the B+ down a bit with a bucking transformer and use a totem pole.