That website, and those pages, are what I used to decode his truck at the beginning of this post. I declared it a 1T because it is what the documents/website say it is. Mr. Rob English lists the page that includes 280 series as 1/2, 3/4, 1T. I think it was assumed all 1 1/2 T, and up, were on the next page. The rest of the internet seems to say that a 280 series is a 1.5T as Mike and the lug nuts say.
I'm away on an ego trip. Will be back on Feb 30. I'm not an Auto Mechanic, but I play one on TV. I charge $0.02 for every opinion and I take Paypal. Plan B is always better than plan A, by definition.
Mike, Your truck is a "Rare" 1 1/4 ton truck. Not many of those were made. You'll notice it has the same small truck grille, as the 1/2 ton, through 1 ton trucks, with a extra piece attached to the splash panel below the grille, to fill the space between the grille, and the channel bumper. Everything else looks like a 300 series, 1 1/2 ton. The 300 series, and larger trucks, have a larger grille. Here's a 280 series 1 1/4 ton, next to a 300 series 1 1/2 ton. A friend of mine owns these two trucks.
Spanky Hardy Collector Of Fine Old G.M. COE Trucks & Antique Holmes Wreckers