Mont. Code Ann. § 85-8-623

The collection of any assessments made by the commissioners for construction and confirmed by the court shall not be restrained or obstructed by reason of any omission, imperfection, or defect in the organization of any district or in any proceedings occurring prior to the order confirming the assessments of benefits, but such order shall be conclusive as to the regularity of all proceedings relating to the assessments of benefits unless appealed from within 30 days after the entry of such order.