Need help here (GMC pickup V8 trivia).

I was looking through some GMC truck literature from the late 1950s and noticed that pickups were available with a 336 CID V8. The engine sure looks like a Pontiac V8 and indeed, GMC had used 287 CID, 316 CID and 347 CID Pontiac V8s in '55-'57 (same displacement as the Pontiac car engines). I'm wondering if the 336 CID engine used in '58-'59 was just an oddball version of the Poncho V8 that was used exclusively in GMC pickups...