my father came from Lublin in East Poland, his name was given to us as Boris Cusazc
I looked at this name again and ... Eureka! Borys Kuszcz could be a quite probable name of your father. Database "Moi Krewni" shows a small number of "Kuszcz" people currently living in Poland: 7 - in the district Biała Podlaska, 3 - in the city of Lublin. One of these people, "Konrad Kuszcz" is registered as an owner of a building and renovation company. Here are all his recent details: edg.lublin.eu/rejestr/?page=41&field=numer&order=desc&akcja=view&id=29095
The word "kuszcz" is of ukrainian origin and it means "krzew, krzak" in Polish. This word apparently also exists in Polish and it was first borrowed by Polish poet Słowacki. Google translates it as "shrub, bush". Family names "Kuszcz" and "Kuszczycki" belong to Polish coat of arms "Kusza" (Crossbow) - if you can prove it of course. :-)
"Kuszcz" was also and administrative and military unit of OUN - Organization of Ukrainian Nationalists. The smallest unit of OUN was Stanica, which consisted of 1-2 villages. Higher unit was Kuszcz, which included 4 to 7 villages. For this reason the OUN's kuszcz has a very bad connotation in Polish WWII history. On the other hand, some Ukrainian officers were contracted in the years 1928-1939 to Polish army. One of them was Wiktor Kuszcz, pl.wikipedia.org/wiki/Ukraińscy_oficerowie_kontraktowi (only in Polish wiki).
There are quite a lot of Ukrainian surnames Kuszcz, some of them quite famous, such as Natalija Kuszcz - a pole vaulter, Anatolij Kuszcz - a sculptor. Some such names might have been transcribed as Kushtch, and this is where a source of the corruption "Cusazc" might lie.