I just ran into this same 'issue' while swapping a 1990 truck rear axle into a 1985 pickup (for the extra width and bigger brakes) In my case I am also doing a spring swap with a thicker than stock spring pack.

The 1990 (and newer) rear axles have a wider spring perch and wider u-bolts to match. The spring isolator 'clamshells' around the spring pack and the u-bolts fit around the entire assembly.

Looks to me like you must use the spring isolators and the wider u-bolts since the narrow style u-bolts won't fit around the spring perch.

My plan is to just reinstall them, they may not fit completely together on the thicker pack, but it looks like they will still work okay.

Anyone else run into this one?
Greg