I used to be afraid to fight against HT in my IS 6. (Still do now, but better.) I can't pen most of my shots even in a T8 match. The nightmare tank for me is another IS6 on the enemy team. (I only brought 3 premium ammo at that time. What a potato) They seem to be able to pen me every time, but I can't return fire effectively with the standard ap shells. Even the frontal weak spot is about 180 mm thick with proper angling.
There are 2 places where I like to shoot at them now. (Of course if I decided to shoot premium ammo, which I do most of the days now against an IS6 and I do recommend you to fire premium ammo at the "balance" soviet steel, it is another story) The first choice for me is the top of the turret. The IS6 has a low profile, this means you can shoot down at it most of the time. The armour on top is 30mm thick and you can overmatch it with most of the gun at t8. (BTW, heat doesn't overmatch. Overmatch means guaranteed penetration) Even you miss the top some how, most of the shells will hit the commander hatch behind. The two hatches are quite large and is only about 150mm thick.
The other place I like to shoot is at the right centre part of the gun mantelet. The gun mantelet itself is 150mm thick but there is no armour behind it at the right centre part.