KE diffs are borgwarner type and s series diffs are salisbury type so the centres aren't interchangeable. chuck up a pic of your current diff, the original tailshaft and your new AE71 item.
And yes the AE71 diff is identical to a KE70 so you either have a different diff in your panelvan or you've been sold an AE86 tailshaft which would have the s series diff bolt pattern on it.
Hope that helps
Dillon