Some thoughts....
Do you know exactly which axle you have, by the markings on the axle ? Just to be sure of the correct specs.
Do you know what your actual axle load would be at the end of the project ? To see how much you are "under-rating" your axle.
Per the build sheet from the manufacturer, it is as follows: "SGL RA 19K 190 WB DANA 19060S Single Reduction, Hypoid Gearing, 19,000lb capacity, 190 wheel ends." Later in the spec sheet is shows "no-spin differential for Dana Spicer (previously Eaton) Rear Axles."
SGL RA is single rear axle.
19K is the rating in pounds. It varies on Dana's site from 19K to 21K for this axle, but the build spec. is 19K.
190 WB should be the vehicle wheel base the axle is designed for, but the actual wheel base is 171.
No idea what 190 wheel ends means. 190mm is 7.5", 190cm is 74.8". That may be it, or it may be something else. They use SAE for everything else, so I can't imagine them using metric.
I also have the ratio elsewhere of 5.29, so that is the ring/pinion ratio.
I know that this is unchanged. The axle has not been swapped or otherwise modified.
The data plate had the vehicle at 29,000 GVWR from the manufacturer. I had the vehicle derated on the door jamb (yes it is legal) to 26,000 so that I don't require a CDL in the state of registration so long as it is a private registration. It is essentially already derated 3K pounds. I have yet to take it to a scale, so not sure how much I have to work with. This is all pre-build out, so doing research at this point.
The way I read the specs, they want you to mount a single tire with it's centerline or track at 72. This would give you an overall width of 84" if you use a 12" wide tire.
If you use a wheel that moves the track wider, you increase load on the bearings; that's why I'm asking the first two questions so you can "judge" the effect of this offset.
I calculate this slightly differently. If the tire is 12" wide, it's virtual center is at 6". 72" centerline track puts the outside tire width (outside tire wall to opposite outside tire wall) at 78" vs. 84". I certainly like the numbers they way they come out in your calculation more than the way they come out in mine. Note: looking at this again I realize that you are calculating both tires at 6" outside, for a combined 84" distance. Now we are on the same sheet of music. I'll be up at the new place I am moving to (where the truck is located) next week. Should be able to re-measure and confirm then. Hopefully that takes some of the guesswork out of this. I think if I get the offset right, simply flipping the wheel for dish in vs. dish out is the way to go vs. spacers. Spacers could be used to fine tune that distance if the limitations of what is on the market (the offsets available) are just off the mark. It all really comes down to keeping that centerline track.
Thanks for making me do some more mental gymnastics. Sometimes we get caught up inside our own head, and need a fresh perspective to help dislodge and get things going up there.