The Program of the National Fascist Party can be read in the chapter 3 of A Primer of Italian Fascism, edited by Jeffrey T. Schnapp.
I couldn't find the PSI program online; but the party was notoriously led by its left wing, the "maximalists", which makes quite unlikely that there would be many similarities with the Fascist program. They would have favoured a completely different rhetoric, based on class struggle and abolition of private property, not the recognition of the "social function of private property" or an education aimed at "physically and morally shaping the future soldiers of Italy".
The Fascist program makes quite a point of declaring fascism a "new" ideology, breaking with the ideologies of the 19th century, liberalism and socialism. Nationalism (actually, a particularly militaristic brand of nationalism) isn't a "clause" in it; it is a main theme, that structures the whole program, alongside with the idea of "corporations", which seems as removed from classic socialism as much as nationalism is.
So I suppose you were mistaught. The program of the fascisti isn't similar to the SPI program, nor it could possibly be: the whole functioning of the party, and its actual appeal to its members, would be impossible in the basis of "the SPI program plus a nationalist clause".