Here's the kicker, the trucks (& some vans) from back in the '60's up until '84 had a 4" pinion offset. Years ago (before I did rears) my friend had a guy do a 9" for his Chevelle. He started with a truck rear, measured the same amount off of each side & had the axles cut & re-splined. The upper control arm brackets were something to see, one was way down on the shoulder & the other was almost up on the top & yes, the pinion was 4" to the Right (3" more than GM had it), the driveshaft was very near to rubbing the tunnel toward to back (I believe there was some hammering involved for clearance there).
Back to your project, I'm not aware of any oddities with the Strange spool (Mark Williams has a different spline location inside) but I would like to see you have it or at least check with them (maybe someone on here can tell us they swapped one in at a later date) to confirm that it is a direct replacement for a stock carrier in respect to the spline placement.
What I suggest you do is put complete center on a bench & install stock axles without a housing (make sure they are fully engaged). Now measure out from the pinion (in your case 15") with a square & make a mark on each axle. Now if you pull the axles out & measure from the line you made to the spline end & you have the total length for each axle. Of coarse there are other ways to do this, but this is the easiest, no figuring, way to do it. Post up the lengths you get! If
you don't want to get the spare junk, we can do it another way. From this point, since your using big Ford/Strange brakes, your axle set-out will be 2 1/2", so you would subtract this from each axle from the line you made. This will leave you 25" right? This is your HOUSING width. Put your dummy pig in your housing & measure 12 1/2" each way from the pinion, mark the axle tube. Now put your bearing end on the line & mark off the thickness of it...this is where you will cut the tubes, make sense? If not speak up now. I hope this all makes sense, I'm not a teacher or a writer. Talk to you later, Lorne