Here I am with an FX-8320, OCed to 4.640GHz at 1.5V with a CoolIT Domino CPU cooler, though I still need a bit more tweaking on the voltage and LLC. Yes, the VCore is pretty high, but that's what happens when you lose the silicon lottery.
I use different stress testing programs, including Prime95, OCCT, AIDA64, and IntelBurnTest. But another program I've been using lately is called y-cruncher: A Multi-Core, optimized Pi calculating program that is ridiculously much faster and efficient than SuperPi or PiFast. It has a built-in stress test using 4 different algorithms, and supports AVX instructions in the latest versions.
Whether or not you decide to use y-cruncher is up to you. It can be just as stressful as Prime95, if not more, at certain times. I mostly use it to stress out my RAM and the IMC. Your mileage may vary.
I've seen my CPU cores go as crazy-high as 80C [+/- 65C-70C on the CPU socket] while encoding with Handbrake, just to see how really comfortable this CPU is with very high VCore voltages [1.60V], and so far, it's still alive...lol! But please, don't try that at home.
I say with your current temps, you still got JUST a little bit more breathing room, but yeah...lower voltages and temps wouldn't hurt since AMD chips like the cold. See how far you can go with 1.375V and a slightly higher LLC to prevent VDroop/VDrop. If you're stable at that voltage, then you might have a golden CPU.
I'd say if your machine is still alive and kicking without errors for at least an hour then you should be good. For me, I usually run a stress test for about 12 hours overnight just to be absolutely, completely sure. If it can pass that, then the overclock is good to go.