[AROWHED]

"...the GP is a cool luxury car, not a hot rod."

I disagree. The GP can easily be both. A stock 65 GP is around 600 lbs heavier than a 65 GTO, but it's also a better looking car overall, roomier, handles well, has larger brakes, and when it comes down it any car can be built to be fast. It was Pontiac's top model...not just a LeMans option. Grand Prix's were Pontiac's answer to the high-selling Impala. The lack of large collector demand only reinforces the individuality of owning a GP. I've never seen another 65 GP at a car show that wasn't a Pontiac-only event. I get people asking me about mine all the time...at the gas pump, grocery store parking lot, even at a red light. Yes I put crazy wheels and huge tires on mine, but so do Goat owners.

Your end goal for the car really determines this, but if street performance is something you desire, your primary focus should be in the direction of increasing low and midrange torque, not necessarily high rpm power...it takes a lot to get 4400 lbs off the line and moving. Once they are actually needed to further enhance engine performance, headers for first gen Firebirds will usually fit, as both cars are rear steer.
Fabcraft is now making 3.55 rear gears, and Engineered Performance can convert your stock open differential into a posi. Upgrades to the trans as increasing power levels demand.