PCI-SIG warns about utilizing 8-pin to 12VHPWR adapters. They may get dangerously heated. 12VHPWR can power heavy video cards. 16-pin connection carries 55 amps, or 600 watts at 12 volts.
The older power supply's won't have a 12VHPWR connector. Therefore you'll need an adaptor. RTX 3090 Ti cards come with a 3x 8-pin to 12VHPWR converter. However, 8-pin connectors can only take 13.5 amps (more than 150 watts). With a 2x 8-pin to 12VHPWR adaptor, twice as much power will pass with a higher load (not factoring in peaks). High-end video cards need an ATX 3.0 power supply. The RTX 4070 is supposed to utilize 'just' 300 watts, which you save with two 8-pin connections.
PCI-SIG warns; 8-pin to 12VHPWR adapters can get too hot