Sorry to tell you this, but it is highly unlikely what you have now will work at your elevation. I've done the 915 head in the 10s for CR and there is no quench/phenolic spacer/cold thermostat/cold plug, cold manifold /slow curve/loose converter/tall gear that will fix the problem.

I had a friend in your situation just have a portion his 2355s milled with a step to reduce the CR but kept the quench. Ran pretty good.

If you want to try one of the options listed, the 0.060" gasket will be your best shot at getting it to work. You will not get better results by increasing the CR hoping the quench thing will help. You lose ground faster with the increased CR than help from the quench. Again, this is from experience.