The several boroughs, townships, or incorporated towns, in this Commonwealth, who shall enter into a contract or contracts with the county commissioners of their respective counties for the joint construction of improved highways under the provisions of this supplement, are hereby authorized to incur such indebtedness, and issue interest bearing bonds therefor, as may be necessary to pay their proper share of said cost of construction as agreed in said contract or contracts; subject, however, to the constitutional limitations of such indebtedness.
53 P.S. § 1833