The splines for the output flanges on a 928 are in the right place but the bigger/T2 size are they not? And centre is centre, we need to know more about what we can't see...
I think bigger bearings, bigger spider and bigger gerth.. 928 doesn't have the space constraint of the rear driveshaft above it. Which is probably common to early VW30 trans +/- a few mm.
T1/T3 diff is a good choice
Matt