I found out that the engine in my GTO is a '74 pontiac 400. My question is what is the difference between these 400s and the ones from earlier in pontiac's youth? I know that mine is a 2 bolt main and not a 4 and that the heads are completely different than anything 70 and prior resulting in lower power. All of the information I have gathered searching the darkest depths of the internet lead me to believe that, other than the heads and the fact that it's a 2 bolt main, there's nothing different in the guts of the block. Is this true or am I way off?
Don't be afraid of a 2-bolt main engine. Pontiacs aren't ::cough spit:: chevys (thank goodness!) so the shortcomings and failings of the latter don't apply to the former. For example, Pontiac cranks are shorter in length and therefore more rigid and stronger, even the cast ones. I'm running a cast crank in my 461 (stroked 400) and it's going to be plenty strong for the horsepower level this motor's likely to ever see. I'm at 495 now and would need different heads and more cam to push it higher.
More important than the number of main bolts is the specific block casting number (6 or so digits on the passenger side rear of the block) - because different ones have differing amounts of metal thickness in the main webs - and that's where the strength comes from. It's also the reason that the 326-350-389-400 family blocks are generally stronger than the 421-428-455 family blocks - the 3" main journals result in more "meat" in this area than with the 3.5" main journals. When a heavily "leaned on" 455 block fails, it usually cracks up through one of the main bearing saddles. Depending on whose opinion you trust, a "good" 2-bolt small journal block with a quality cast crank is quite viable up to around 600-650 "or so" horespower.
Cylinder heads --- with the notable exception of the 1967 670's, most later model Pontiac combustion chambers are all very similar in shape. Where you're going to find most of the differences are in the chamber volume, intake/exhaust valve diameter, and screw-in vs. pressed in rocker studs. For performance applications you're going to want the 'big" valves i.e. 2.11" intakes and 1.77" exhausts. For strength you want screw-in rocker studs, however the factory screw-in "bottle neck" studs aren't that great either, especially with rowdy cams and heavier springs because they tend to break at the neck. The hot tip is to bite the bullet and have the heads machined to accept the larger/meatier 7/16" studs anyway, so it doesn't matter much what the head started out with --- however 'generally' the larger valve heads also usually had screw-in studs. Combustion chamber volume is one of the key determining factors for static compression ratio - so that's going to matter based on cubic inch displacement, cam profile, intended use, fuel type, etc. So first you've got to make all those decisions, then see what compression ratio you need to be at to meet those goals, then pick the combinatoin of chamber volume, piston, deck height, and head gasket that gets you there. I guess what I'm trying to say is that just because you may have small valve, pressed-in stud heads now -- that doesn't mean they're "bad". Depending on where you want to "get to" with the motor they might be ok with some minor modifications. Aside: always measure ("cc") the combustion chambers on any heads you're planning to use because Pontiac heads are known to vary quite a bit from the published factory specs. for example, the spec for #62 heads says they have 72cc chambers, but actual measurements for a specific head can vary from that by several cc's, and that makes a significant difference in compression ratio.)
Generally the right way to do it is to design your engine set up based on your goals, intended use, and budget --- then start putting together the combination that gets you there. I really enjoy doing that
It helps to have a good engine modeling software package so you can "try" various combinations before you start spending money for parts, too.