Also it's going to depend on your case and your other components. A graphics card that moves all of its hot air out of your case may buy you a bit more room on the upper end than would one which dumps part of its hot air back into the case.
Some cases are pretty easy to modify in terms of adding a case fan blowing cool air right down on the CPU fan and heatsink. I don't know if this is what you might also consider "custom parts," but it might go a long way towards a bit of a safety margin for your CPU.
Even better, two case fans, one blowing down on the CPU fan and heatsink assembly, and another blowing across it as well. The more cool air you can make available, and the more warm air you can move away, the better off you'll be.
For a stress program for CPU, I use AIDA64's "system stability test." I have AIDA64 anyway for its monitoring features, and its stress test seems to work well.
Best of luck on tuning your system and getting it the way you want it.