I support your conception of a low grade sarcoma and favor a sclerosing epithelioid fibrosarcoma with a low grade leiomyosarcoma or myofibrosarcoma as diff. diagnosis.

For proper interpretation an XRay or at least the precise anatomical location would be mandatory. Where is the main part of the tumor located? Is it projecting into the pleural cavity? Although the morphology is not very typical, solitary fibrous tumors can look weird and quite often occur in this location. Also synovial sarcoma should be ruled out (basic immunohistochemistry against cytokeratin). I agree that sclerosing epitheloid fibrosarcoma is a differential diagnosis (very rare), leiomyosarcoma and myofibrosarcoma are (in my opinion) rather unlikely. In any case, treatment should focus on complete resection of the tumor with clear margins.