You are completely wrong. Every standard Pentium motherboard made that supports 2x multiplier will work with the K6 series at 400. The only ones that will not are specialty boards from Dell and maybe Compaq. Some boards do not allow split voltages, which means that they require a voltage step-down converter. But I have run almost every type of standard dual voltage motherboard made with the K6-2 at 400 or 450 (depending on the limit of bus speed) using the 2x=6x trick. Even at 2.8v the 2.2v K6-2 works fine, if you properly cool it. But he has the voltage he needs, so I can say with confidence that it WILL WORK!
Cast not thine pearls before the swine