Live in high altitude (8200 ft) Colorado, what changes must be made to make this cake?

I feel your pain; baking at high elevations is a challenge. That said, it's certainly possible to make your results more dependable. Essentially, you want to decrease the leavening because at higher elevations, there is literally less atmospheric pressure bearing down on what you are baking, so baked goods rise much more easily. Second, you need to increase the protein in the forms of flour and eggs so that their structure sets up more firmly, quite literally holding your cake or muffins up. For your elevation, decrease your baking powder by 60%, increase your flour by 10%, and your eggs by 15%. Leave baking soda measurements alone; baking soda is proportional to the amount of acid present, but do increase flour and eggs. And also increase your baking temperature by 25 degrees. Please let us know how this works for you.

The high altitude baking adjustments I have followed for 6,000 to 8,500 feet are as follows:
Baking Powder: Reduce each teaspoon by 1/4 to 1/2
Sugar: Decrease each cup by 2 tablespoons, unless the recipe calls for less than a cup, then no adjustment
Liquids: Increase each cup by 3-4 tablespoons
Baking Soda: Decrease like baking powder. When acidic ingredients such as sour milks or fruit juices are involved no adjustments
Fats: No adjustments
Oven Temperature: Increase temperature by 25 degrees.

All that being said, the only sure way to know what to do with each recipe is to experiment.