HJ buckets won't fit the WB bolt holes either. Keep an eye on using earlier series stuff in WB as they have a lot of metric bolts such as seat belts and seat mounts.
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I know it's a bad resolution photo and this was in 94, but V5 Simmons on a 83 WB ute. So I guess bigger size rims will look similar. Personally I don't think 20" wheels really suit older style cars. But the beauty of cars is to build them how you want to.

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The sedan diff is a coil set up. You would need to mount leaf spring mounts to the diff. Can be done but easier just to use a ute/van diff.
Discs open you up to other issues with the hand brake as well. Plus you need to change the master cylinder to suit. It's all been done and is quite do-able.