Yes a 89 will be a KP axle, they swapped to balljoints in 91.5. The rear will be a steerling 10.25, not my favorate axle but very strong and the after market world is catching up to them.
Is that F-350 a single wheel or DRW truck? The front is the same ( DRW VS SRW ) you just need to swap out to a SRW hub if there DRW one's.
Brakes I run the stock one's on the front 60 and then GM 1/2 tons on the rear, TJ master. Works fine and I can lock up 39's with little effort.
Most diesal trucks will have 3.55's, it could have 4.10's but most trucks will be 3.55's from Ford.
If you want to narrow a them I woul dstart with a GM 14 bolt insteed of a steerling, why you ask? Well you can get a DRW 14 bolt ( or a SRW 14 bolt and swap the hubs ) and it will be 63"s WMS without cutting a tube or shaft. The steerlings used all one hub and shafts.
The steerling use's a 35 1.5" spline shaft ( like dana )very strong, but its cut at a different pressure angle wich makes it harder to have a respline shaft made cause its tooling most axle guys dont have yet.
Wish you were closer I have a narrowed ( 64.5" ) 88 Ford with 4.10's, lock right locker and PSC high steer arms on it for sale,. Thats axle served me well for years, just moved to a 78 front 60 ( wanted fullwidth ) so the 88 gut pulled for the 78 axle.
Your making the right steps in thinking one tons, if the end idea is a big tire jeep this is way to start.
Its a SRW, and I believe that the gears were an option that they got on the truck (for towing). But I don't really care what the gears are anyways, since I'm planning on swapping them out later for 4.88's or 5.13's.
Anyways, I did some looking around and it seems that the sterling 10.25 isn't such a bad option. From what I have been reading, it has better clearance than a d60 and is comparable in strength to a d70. A little overkill for now, but I'm sure that will change. I found rear disc conversions for it with a manual parking brake from blackbird for around $550. Which doesn't seem too bad. As far as resplining the axles, do you know if it is possible to change out the gears on the carrier so it would accept a more common pressure angle? I guess my first step should be calling around to a few shops near me to see if they have the ability to cut the correct splines.
I've read through a few builds and it seems like there are a few small like problems carrier bolts coming loose, tubes twisting, and more of a limited selection of gears. But I'm thinking if I can find a shop that can respline the axles that I'm going to give the sterling a shot since I already have it.
Yeah it is too bad you live so far away, I would be on that deal in a heartbeat. You wouldn't be interested in parting out the high steer arms or the flat top knuckles would you? (im guessing you have flat top knuckles)? I think I know a good home for them