I have an Output Enablers socket in my Quadra 700 that allows me to swap out clock oscillators and I cannot get my system to boot with anything higher than a 74 Mhz oscillator which equates to a 37 Mhz speed for my CPU. I suspect the issue you are seeing is because the other hardware on the Quadra 700, beyond the CPU, cannot support 40 Mhz. As a result, it won't boot. I use a 66 Mhz oscillator (for 33 Mhz CPU) as it is the most stable and doesn't freeze up like I often get with the 74 Mhz oscillator.