Mopar made almost yearly changes to truck steering in the late 70's. Most notably the mounting of the box for 4wd. Also steering column changes mainly the coupler.

That said pitman arm are non wear items. Differences are manual v power, 2v4 wd, early or late 4wd.

Having not dabbled much with 2wd trucks I wouldn't rule out a steering redesign in the late 70's that would require a different arm.

All in all though it shouldn't be hard to find a good used or nos part once you nail down a part number.