If you do a search for Commodore V6 you'l find this topic has been discussed many times. You'l also find very negative opinions of such a conversion,
Seeing as you've already got the vit engine, do it! Commy conversion=crap IMO.
luke3411 wrote:
Have many people done the holden 3.8 conversions and who would think this to be a good idea? the3 extra power would be awesome and I do realise I would have to increase the capacity of the fuel tank to stop it giving me the shits

Fuel tank wouldn't be much of an issue (well, you'd have to mod it for EFI anyway), it's more that the engine weighs several times what a normal zook engine weighs. Also, with all the extra power, you're just going to brake everything.
What are you using the car for?