Multiple Myeloma: The presence of a large new lung mass in conjunction with significantly elevated monoclonal protein (18 g/L) and an abnormal serum protein electrophoresis (SPEP) with an alpha-2 globulin level of 14.2 g/L strongly suggests multiple myeloma, especially considering the age of the patient. Multiple myeloma can cause a variety of systemic symptoms and can lead to the development of masses or lesions in various parts of the body, including the lungs, either directly or through associated conditions like amyloidosis.

Waldenström's Macroglobulinemia: Although less common than multiple myeloma, Waldenström's macroglobulinemia is another lymphoproliferative disorder that can present with monoclonal protein production. It typically involves the production of IgM monoclonal protein and can cause a range of symptoms including hyperviscosity syndrome. The lung mass could be related to the disease process or a secondary infection.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lung masses and abnormal protein electrophoresis due to monoclonal protein production. The age and the specific lab findings would need to be considered in the context of lymphoma subtypes and their propensity to produce monoclonal proteins.
Chronic Lymphocytic Leukemia (CLL): CLL can sometimes produce monoclonal proteins, although it is less commonly associated with large lung masses. However, CLL can involve the lungs and cause masses or infiltrates, especially in advanced disease.

Metastatic Cancer: While the monoclonal protein suggests a hematologic malignancy, it's crucial not to miss other types of cancer that could metastasize to the lungs and potentially cause an abnormal SPEP due to paraproteinemia associated with some solid tumors.
Infection or Abscess: Infections, including abscesses, can present as lung masses and must be considered, especially in elderly patients who may have compromised immune systems due to underlying conditions like multiple myeloma.

Heavy Chain Disease: A rare condition characterized by the production of abnormal heavy chains of immunoglobulins, which can present with a variety of symptoms including masses or lesions in different parts of the body.
Amyloidosis: While not a primary diagnosis for the lung mass, amyloidosis can be associated with multiple myeloma and monoclonal protein production. It can cause a range of systemic symptoms and organ dysfunction, including potential lung involvement.
POEMS Syndrome: A rare paraneoplastic syndrome associated with a monoclonal plasma cell disorder, characterized by polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy, and skin changes. Lung masses are not a primary feature but could potentially occur in the context of associated conditions.