At 490+ defense, you should be crit immune, so big hits like 7k should be rare. That said, the Underbog Lords at the end of Heroic UB hit HARD. They get a stacking buff that boosts their damage the longer you take to kill them, and I’ve seem 9k+ hits from them. Even then, they hit slow enough that a healer spamming their biggest heals should be able to keep you up long enough for the dps to kill him, unless the dps are very weak.
That said, Heroic UB is hard on the tank. CC is important in heroics, unlike normal. You need much better gear (full Phase 1 BiS) before you can comfortably go without CC. I’ve gone without CC in less gear than that, but I also had cracked dps that annihilate things. A mage with improved Blizzard is invaluable, as that slow allows you to back out and give your healer a chance to catch up.
As for gearing, you have the “benchmarks” I’d get before doing heroics (490 def, 10k unbuffed hp), but I do have to admit, your avoidance is low. With Holy SHield and buffs, you’d only be at around 75%, 80% with the badge trinket. That’s really low. With only a couple pieces from heroics, I’m sitting at close to uncrushable (101.6% buffed). You don’t need uncrushable for heroics, no mobs in their can crushing blow you, but that avoidance/block really helps out healer mana. It also helps your threat, as blocks = threat for a tankadin.
Big Pieces to aim for to help you get there: Cloak from SV, Chest from Mech, Legs from revered with CoT, shield from Slabs. That’s all I can think of off the top of my head. Tanks actually only have minor upgrades in heroics (though badge rewards are GREAT).
TL;DR, make sure you’re using CC when you can, especially “soft” cc, like hunter trap and imp blizzard, and seek out the last few upgrades you can get from normal dungeons. Gem for Stam unless the socket bonus is stam.