I'd go ahead and set all cores to 40x and give that a try at stock volts. Run some stress testing app and see how it does, if it passes, you can bump up the multi 1x until it fails, then take it back 1x multi. Stock volts should be around 1.25 or so, you have a little room to increase it, but not a lot given your cooling. You'd probably be ok 1.25-1.4V or so.