I need some insight for guys experienced with the 3800 platform. I am a first time owner of a 2005 gtp. Its currently stock and im not sure where to start my build. any thoughts or suggestions? Like what plugs to run, how much HP stock internals can handle, ect.

We just happen to have a thread on that:

https://www.gmforum.com/performance-brainstorming-tuning-96/what-order-should-i-mod-my-engine-278662/

Let us know if you have any more specific questions after reading through that thread. Members of the forum have explored just about every possible performance enhancing option available to these motors over the past 10 years and we will certainly be able to give you the advice you are looking for.

Yep, what Tom said^