: "Apologies for the gen1 / gen2 comment. I did get that from the wiki guides and posts flying around the internet. I'm used to that talk from sports cars as well so it kinda stuck."
: Anyone can add to Wiki and it is obvious that a non-GTO person wrote the article. There are ONLY 2 Generations of GTO's if you gotta use that silly millennium generation term. 1st Generation GTO 1964-1974. 2nd Generation GTO 2004-2006.
: "I'm trying to find out more information on the 400 and 455 engine. If it's been rebuilt, do you recommend doing a compression test at a mechanic before buying to make sure it's not above 9.0?"
: Keep in mind that your experience with Porsche should not be confused with Pontiac - apples and oranges in comparison. You may be highly disappointed in the heavy, lumbering, and poor handling/braking attributes of the GTO as measured by the Porsche experience. My guess is that once you get your seat in one, you will want to do a number of upgrades in an attempt to "fix" or improve these issues. This can be done to some extent, but this will cost additional $$$ above your projected budget on the car's purchase.
Pontiac offered their engines in a range of compression ratio's from the factory. The GTO and/or high performance engines were the engines with the higher advertised 10.5-10.75 compression ratio's. Actual out the door compression ratio's were up to 1/4 a point of compression less. The optional engines could be ordered having lower compressions. Compression ratio's dropped after 1970 to meet emissions requirements, so if someone has dropped in a later replacement engine, it may have a low compression engine.
Doing a compression check may or may not indicate a lower compression engine. The camshaft events can have an effect on the dynamic compression and you can have a lower compression reading on a higher compression engine. So a compression check can be used to give you some idea of the engine's compression ratio, or even its condition, as you know.
Generally, the way to lower compression on a Pontiac engine is done 2 ways, piston cc's or cylinder head cc's. Pontiac used flat top pistons in all their engines and varied the compression by opening up the combustion chamber and adding more cc's to lower compression. So you may find a factory high compression engine not having its original factory heads and substituted with a set of later model heads having larger chambers to lower compression. The second method is to add more cc's to the valve reliefs or use a dished piston with the original numbers matching heads of the high compression GTO engine. These are aftermarket forged pistons readily available from several sources. This is preferred with original numbers matching engines/heads and cars.
Pontiac also used the same engine code letters multiple times over the span of their engine production, so identifying the engine by this 2-letter code may not always be accurate. Heads have a 2/3-digit code on the center exhaust outlet to identify them. There are also casting dates to verify. Most 400CI engines built before 1975 are good, the later 1975 blocks are weaker and not good for high performance builds - unless a Trans-Am engine with the "XX" casting.
The 455CI will most likely be a low compression engine IF stock. Again, most will be tweaked on the side of performance and compression boosted. You will find a much higher percentage of 400CI engines over 455's. Again, same with the head cc's and/or piston dome selection.
In my opinion, trying to find that "perfect" driver may be tough. You don't really know the condition of the engine whether it is original or rebuilt unless you have the documentation for review. If you get an original car/engine that has not been apart - you will get a used engine and you do not know its condition internally. Most were "used" hard unless it was a documented car owned by a little old lady who never went above 50 MPH. Timing chains used a nylon teeth on an aluminum gear up to about 1971 and wore out by 60,000 miles. The oiling system is sited as a problem, not because it is poor, but in my opinion, those hard launches most people do with a GTO sloshes oil and may uncover the oil pick-up screen and suck in air momentarily. Eventually rod bearing wear takes its toll from both higher RPM's and an occasional oil starvation second or 2 - and lack of regular oil changes. But, you won't know this without inspecting the bottom end.
I suggest you do all you can and take the time to study and learn about these cars before purchasing one. I would set my purchase date about 1 year out so I could get a good learning curve under my belt and select a purchase that meets my needs. It may be a factory original car, a "resto-mod," or a Lemans/Tempest body that fits the bill.
Purchase these books, Pontiac Muscle Performance 1955-1979
by Pete McCarthy, and Pontiac GTO Restoration Guide 1964-1972
by Paul Zazarine. This will help to answer a number of your questions and familiarize you with the GTO - 1st Generation.