Rear discs are really only for high performace, high speed work. Will never happen with a WB van, so a waste of time and effort. Standard drums are perfectly suited to the task, would be a waste of time, money and effort to fit HZ/WB discs. Not sure why you'd want a VT booster either, standard WB is more than up to the task.

All Holden 6cyl are the same bellhousing pattern. It is VERY different to a V8 manual or auto (TH) engine. To convert a WB 202 with Celica 5spd to a Holden V8 engine will require:

CRS or Dellow bellhousing, pull clutch style (full cable) for Holden V8 to Celica.
V8 pull clutch fork (late HZ, WB or VB-VK).
WB V8 clutch pedal (different to 6cyl).
Modified tailshaft as the Celica box has to move forwards about 30mm or so (by the difference in length of input shaft between 6cyl aussie 4spd and V8 4spd).
Modified crossmember.

The stick may no longer come up in the right place so may have to be modified. Tailshaft won't be an issue as you'll probably upgrade to a V8 diff anywayas WB 6cyl Salisbury is just a banjo in disguise. From memory with a Celica box behind a Holden V8 you can use the standard V8 tailshaft with special gearbox yoke.

Then the first time you give it a bootful the 308 will chew the guts out of the Celica box and you'll have trouble finding another one! Much better idea to use the trimatic out of the WB Statesman, heaps better box, easier and cheaper to fit and if looked after will last forever.