A resolution of issuance may include provisions establishing the amount and terms of purchase contracts to be held in trust in relation to the revenue bonds that are outstanding, including the setting of both maximum and minimum required levels in terms of percentages or otherwise, and such provisions may also:

1. (a) Prescribe the method for setting the value of any outstanding purchase contracts;
2. (b) Continue authorization for the department to withdraw purchase contracts or other assets in excess of the maximum required level; and
3. (c) Permit substitution of purchase contracts or other security under appropriate limitations.