Jerry,
I posted a part number for that nut, which I think was used on 216, 235, and early 261 engines; however, I removed that post because I think that GMC used a different fitting than was used on Chevrolet engines.
Maybe someone will clarify this. If the GMC and Chevrolet fittings are the same, the nut is readily available from NAPA and others (I'll post the Weather-Eaton part number).
Tim
