If directed to do so by the Department of Revenue, through regulations or instructions upon the state income tax return form, every taxpayer required by this chapter to file an income tax return with the department shall also file with such return a true copy of the federal tax return filed by the taxpayer pursuant to the requirements of the Internal Revenue Code for the same taxable year. The department may, in its discretion, promulgate regulations or instructions that permit taxpayers to submit specified excerpts from federal returns in lieu of submitting copies of the entire federal return. The federal return or any part thereof required to be filed with the state income tax return is incorporated in and shall be a part of the state income tax return. [1969 c.493 § 66; 1977 c.872 § 6]