The Secretary is authorized to sell such lands to resident farm owners or resident entrymen, on the project upon which such land is located, at prices not less than that fixed by independent appraisal approved by the Secretary, and upon such terms and at private sale or at public auction as he may prescribe: Provided, That no such resident farm owner or resident entryman shall be permitted to purchase under sections 424 to 424e of this title more than one hundred and sixty acres of such land, or an area which, together with land already owned on such Federal irrigation project, shall exceed three hundred and twenty acres: And provided further, That the authority given hereunder shall apply not only to tracts wholly classified as temporarily or permanently unproductive, but also to all tracts of public lands within Federal irrigation projects which by reason of the inclusion of lands classified as temporarily or permanently unproductive are found by the Secretary to be insufficient to support a family and to pay water charges.