True, but take the measurement at the wheel hubs rather than to the back of the brakes. I've measured some of these rear ends, and they're actually pretty close, but still an inch or two wider.
I think they would still work because in order to use the year 2000 GMC chrome 8-lug wheels on my 3/4 ton '52 GMC, I had to use 1 3/4" spacers. Even with my wheels spaced 3 1/2" further apart than original, they still fit nicely inside the wheel wells. That's why I think a wider rear end would still work. I'd estimate that you could go at least 4" wider than stock, and still fit inside the wheel wells of a 3/4 ton or 1 ton.
When I convert to the Ford Dana 60 3.73 rear end that I'm planning, I'll be able to put the same wheels on without the 1 3/4" spacers and fit perfectly inside the wheel wells. The only work is to adapt the spring and the shocks to work with the truck.
And yes, the same 8-lug wheel pattern will fit with these GMC wheels. The center hubs are different sizes, but they still seem to fit.
