Other than being able to use one spare for any of the 6 positions, is there another reason to change the front track on a dually by using a dually wheel on the front axle?
If not, I'd probably just leave the SRW front tires/wheels alone and worry about the rear duals. Then later, after you find a nickle in a pig track, convert the front axle too.
In over ten years and over 184,000 miles, I've never had to use the spare tire. I've had flats, but always a slow enough leak that I was able to add some air and make it to the flat fixer place without having to mount the spare. Last week when I replaced the rear tires, I put the best of the 3-year-old take-off tires in the spare and threw away the 10-year-old spare tire. I was surprized that the spare tire winch still works good. Anyway, the point is that I wouldn't be too worried about not having a spare tire for the front axle.