A group of Harvard alumni proposed a solution for how the university’s endowment can pay for a new tax on investment income and boost returns: don’t think so much. Harvard should use the savings from shifting half of its $37.1 billion endowment into low-cost funds tracking the S&P 500 to cover the cost of a new tax that some of the wealthiest schools will face, the group said Thursday in a letter to incoming President Lawrence Bacow.