1965 was the first year for the TH400 trans in big Pontiacs, so you're golden there.
The host of this forum, Performance Years, sells a lot of resto parts for full size Pontiacs. Another source is Ames Performance.
There are any number of high performance parts for the engine, do some reading here. There are several Pontiac engine specialist shops who post regularly.
Nunzi's, at
http://www.nunzi-pontiac-expert.com/headers.htm sells headers for '61-'64 big Pontiacs, don't know if they'll fit a '65. Again, do some searching around on this site, I believe some people have used certain GTO or Firebird headers on big cars with some success.
Good luck. I've always likes '65s, I used to have a '65 Grand Prix.