What errors do you get with Heaven? It does have some artifacts (near the dragon for example) which show on every card.
To overclock higher, you need to increase your voltage. See what you can get with 1300mV core. If this is not enough, then there exists a modified version of Trixx 4.4.0b which will allow you to go up to 1381mV.
Here's the link:
Overclock your core and memory separately, otherwise you will not know what's limiting you.
Effective voltage will be significantly lower due to vdroop, so this is not dangerous, but in any case you should monitor your GPU and VRM temps with software like GPU-Z.
Best stick for <75C core and <90C VRMs. Absolute safety limits are probably around 90C core and 110C VRM, but you're unlikely to have your card stable with these temps anyway.