Leases issued under the provisions of this act [19-8-14 to 19-8-18, 19-8-21 to 19-8- 33 NMSA 1978] shall cover a specified area conforming to a legal subdivision or subdivisions and shall not exceed sixteen (16) subdivisions, being 640 [six hundred and forty acres] more or less, all of which shall be contiguous; provided, however, that leases issued in exchange for existing permits as provided by Section Four [19-8-17 N.M. Stat. Ann.] of this act need not be contiguous if all legal subdivisions be situate and lie within the same township and range.