That's right on rare. I've never even heard of them or seen them listed in any chevy list for this axle. 3.90, 4.10, 4.56, 5.13 I've heard of, but never a 3.54. Are you sure it's a GM and not an International? They look similar.
If (and I mean if) it fits, you'll have plenty of highway gearing... but you can forget about using your truck as a 1 ton. You'll have little low end grunt, even if you have the granny 4 speed.
