Stockpiling Gold for Japanese Anniversary help.

New player, first time with Japanese Anniversary help. I know I’m supposed to stockpile at least 400 cubes and 300k gold. The cubes I have but how do I stockpile 300k of gold if my storage cap is only 51.6k? What’s the best places to farm gold at commander level 76 (10-4 is my highest unlocked level).

Thanks all