The decision of the legislative body shall be final and conclusive, and it shall thereafter be conclusively presumed that the project area is a blighted area as defined by Section 33031 and that all prior proceedings have been duly and regularly taken.

This section shall not apply in any action questioning the validity of any redevelopment plan, or the adoption or approval of a redevelopment plan, or any of the findings or determinations of the agency or the legislative body in connection with a redevelopment plan brought pursuant to Section 33501 within the time limits prescribed by Section 33500.