Are you looking at a C2500 axle, or a K2500 axle? Far as I know, the C and K axles are different widths. Which maybe explains the axle-shaft length problem.
I'm prepared to learn something...but...I have NO idea how the rear wheel track widths can be different, the axle shaft lengths can be different, and the axle housings be the same
width. Look at where the bearings ride on the axle shaft--if the axle tube isn't longer for the longer axle shafts, the bearing wouldn't be in the right location.
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Aside from the 6-lug axle shafts, you'd also need 6-lug brake drums. I don't know why the backing plates would be different. By the time you buy new axle shafts, and new brake drums...maybe it'd be better to just get the 6-lug axle assembly to begin with. Kinda depends on the condition of the axle assembly--are the axle shafts or drums worn-out? If so, you need to buy those parts anyway.
When it was me, I got a "K
2500" 6-lug axle for my K
1500 6-lug truck. Bolts right in. No wheel hassles, no brake drum or axle-shaft hassles. Needed four U-bolts, the U-bolt plates from the donor vehicle to match the axle, and a conversion U-joint (#441, I think) for the rear of the driveshaft.
In my case, I also needed to swap shoes around on the rear brakes because the previous owner of the axle did his own "brake job" when he was clearly unqualified.