As I always tell people, anything can be done it all depends how much money you spend and how you go about it.
For instance, I am converting my girlfriend's 2007 eclipse from 4 cylinder to V6 with a turbo. We bought a whole engine and transmission package and begged them to keep the vitals with it such as the harness and the alternator and starter and stuff. We picked the package up for 1800 and we will complete the swap for under 2500.
To answer your specific question, yes, it can be done. Someone is doing the conversion as we speak to an LS V8. I am in the beginning stages of doing a V8 swap myself with a type of engine not used in G8s at all. If your budget is less than $3k, I would have to say it's not possible. If your budget is closer to $5k or $6k, then it may he possible.
Off the top of my head, you need:
Engine control module
Dash harness modifications
All belt driven accessories
If you want the v8 sell the v6 car and buy a gt or gxp. Huge headache and you won't ever get your money out of it.
-hood vent mod
-camaro trans pan n filter
-removed engine cover
-drilled front rotorpro rotors
-tinted tails and front yak emblem
-Spare set of 19" GT rims
-custom interior trim
-rare roof rack
-smoked side markers
-New steering wheel to install
-New all black cupholders and shifter shaft to install
- Mobil 1 synthetic
There's no question trading it in would save a lot of money and aggravation. However, what if you have modified your car appearance wise and don't want to start over? What if the fact that it is your first G8 is sentimental to you and you don't want a different car? This question has been asked on every car forum I am part of when it comes to less optioned vehicles and people always jump to say trading it in is the better choice. But for some people money isn't an issue and for some people its worth the time and money to still have their car and not another especially if they bought theirs new. I have strived to have the most modded V6 driveline and i do. Not because it was cheap though. I could've bought a GT with the money I've spent but instead I'm going to make my car what I want it to be because it can't be bought on a dealer lot. The way I see it putting a 6.0 or a 6.2 in a V6 car doesn't have the cool factor and to me personally it isn't worth it which is why I've chosen a more exotic power plant. But as they say, to each his own.