Assuming it has a proper HQ-HJ rat-trap clutch setup:

Change gearbox input shaft and cover to V8 M20, or get a V8 box.
The clutch setup is the same for 6 and V8 except for one bit - that bolts to the bellhousing. So you need to find one of those.
Get a HT-mid HZ 253/308 to aussie 3sp or 4spd bellhousing and clutch fork.
Complete 253 or 308 with HQ-WB sump and pickup plus HJ V8 engine harness (might have to make the harness yourself from the 6cyl setup).
HT-WB 253/308 flywheel and new clutch kit with correct throwout bearing and carrier.
HQ-HZ viscous fan.
V8 radiator and fan shroud.
V8 HQ-WB ute or van diff.
Tailshaft from a HQ 253/308 Statesman or HQ-HZ 253 any box or 308 manual wagon or HQ-WB 253 any box or 308 manual ute or van.
HJ-WB V8 throttle cable.
All radiator and heater hoses and clamps to suit.
New exhaust.

The other alternative is to get a mid HZ-WB, VB-VK 253/308 aussie 3spd or 4spd bellhousing and run a mid HZ-WB V8 clutch pedal and cable. WIll need a hole drilled in the firewall for the clutch cable. Requires a different throuwout bearing and carrier in the clutch kit.

Or it might be bettr to buy something like this and sell off what you don't need. You can use everything off this car from radiator through to diff and exhaust. If you are lucky it might be a later 1978 with the right clutch pedal setup.
http://www.ebay.com.au/itm/Holden-HZ...item3a7d6144c5