Does anyone have a tape measure? Go find out... I know very little about superdutys, but duallys have been my thing for a few years... I'd go out and measure mine right now if it was here, but I can't ...
What matters is the (WMS)wheel mounting surface-to-wheel mounting surface measurement. On a dually, it would be as simple as measureing from the outside of the outside tire on one side, to the inside of the inside tire on the other side. (for example, left side of the left tires, to left side of right tires, would give you center-to-center measurements, and thus give you WMS to WMS.) The old chevys I know of were about 69" on the front, stock single wheel trucks, and about 66" on the rears, 1/2 ton thru 1 ton trucks. From compairing my '00 F-350 to my '84 K-30, I'm guessing you'll find the rearend WMS-to-WMS to be about 69-71", the frontend may be about the same, if you dont have DRW spacers, as mentioned in earlier posts. Moving these spacers would push the rear wheels out WAY TOO FAR in my opinion, but I am basing all my knolage on my cab and chassis truck, might be wider than the standard bed models.
From what is sounds like to me, you want to swap a dually rearend into your single rear wheel truck? My guess is that the bolt pattern would be the same, the WMS measurement just might be off... If it's not much, it wouldn't hurt to use it anyway, the older trucks (my chevys) were all that way, 69" front, 66" rear, made the front tires stick out of the fenders when you went with wider/biger tires, rears still nicely under the bed sides... just looks funny. Good luck...