74°C is actually just the "Tcase" specification which means that this is not the maximum safe temperature for the cores. Instead, the maximum safe temperature for the cores is basically the Tj. Max (which is a spec for the core temperatures). The Tj. Max is the "Thermaljunction Maximum" which is where the CPU would either turn itself off or throttle itself down if that core temperature were reached. For CPUs like the E7400, the Tj. Max is 95-100°C.
But for the sake of stability while stress testing an overclocked system, I recommend trying to keep the load temperatures out of the mid to high 70s because this may result in a failed stress test just due to the heat. I mean, excessive heat can easily cause instability when the system is otherwise rock-solid stable. But this doesn't mean that the mid to high 70s are physically unsafe; it's just a general area to stay away from while running a stress test to test an overclock because it may result in a failed test simply due to the heat and not due to instability.