The problem is you're not getting enough thrust from the tail to counter the torque of the main rotor. When you put the longer blades on it increased the torque that the tail needs to overcome. You compensate for this by sliding the servo like you mentioned for more pitch, but if you're so close to the limit that you're stripping the servo then it means you either need more speed or a bigger tail rotor to get more thrust from the tail.

The simplest thing to try at this point is to increase your head speed. I think the stock radio has some knobs for adjusting the pitch and throttle curves right? You need to adjust those to get more head speed at hover and it should help (increase the throttle knob and/or decrease the pitch knob).

I don't know what you would replace the tail blades with to get more thrust.

The tail on the HBK2 is said to be a little underpowered and there's a tail drive pulley upgrade available that increases the speed of the tail relative to the main rotor, but I've never heard of this being a problem with the belt cp before.