A few C isn't going to make much difference, what WILL shorten its lifespan is overvolting, which also dramatically increases the heat output BTW.
Just be aware that stress testing will push the card harder than most games, so expect to see >80C if you run something as evil as Furmark to test it, transient temperatures of 90C should be OK so don't break into a sweat if it trickles over 80C under test (I've had my old HD6950 up to 88 on test with no ill effects).
And don't use OCCT, if Furmark is evil OCCT is positively malevolent!
What about max voltage? What should be the absolute maximum without causing damage?
I think you'll be more likely limited by the temperature rather than the max safe voltage. Once you start increasing the voltage the heat output will go up quite dramatically and unless you're running a custom waterloop you'll be hitting the temp threshold pretty fast.
And if you overclock using amd's control center, it doesn't support voltage control as far as I know anyways, so you'll be stuck on the stock voltage. You might still get pretty decent oc on it though..