Whew, where to begin? As a general matter they're not horrible, but adding big tires and lots of horsepower pushes them past their limits. The fact that yours don't stop straight indicates something's not right. Maybe you have air in one caliper that's causing that side to work less than the other side. You could also have grease or oil contamination on the pads on one side, causing uneven braking. And of course, you could have a sticking caliper that causes one side to work poorly. When set up and maintained right, they'll stop pretty good.
One huge issue that's frequently overlooked is the front/rear balance. These cars use ancient technology drum brakes, and in 25 years of hot rodding I have never, ever, ever (ever) seen the "automatic" adjusting stars keep the drums adjusted right after any length of time. The most noticeable condition is nosediving, which can be eliminated with correct drum adjustment. Maladjusted drums are also a huge suspect in pulling to one side. A lot of people ignore the drums because they can't see inside, and are frankly mystified by how they work. And even when guys do reline the drums, I'm sure that many skimp and don't install a new small parts hardware kit that includes new springs. Old and weak drum brake springs causes poor release when you take your foot off the pedal. And you've gotta adjust them right, since they don't just adjust themselves like discs will when you replace the pads. Just getting the drums at the correct adjustment will make a huge difference. I find that I have to adjust the drums manually at least every 5000 miles.
Just for one mystery war story: I stupidly mis-installed rear drum brake shoes, and had a weird pulling and side-to side imbalance that ate the front pads and disc up on one side in only a few thousand miles, when the other side was fine. I thought it was a sticking caliper, but ultimately figured out that I had gotten the rear drum shoes mixed up -- they come in pairs, and each side gets a small one and a bigger one, called the "leading" and "trailing" shoes. I stupidly put both leading shoes on one side, and both trailing shoes on the other side. It caused the weirdness described above, and cost me a front rotor.
Take a good look at the rubber brake lines too, for puffy or spongey appearance. These can get dry rotted or just worn out on a 15 year-old car. Stainless braided brake lines can be had for $125, and will assure a firm pedal. Mine work pretty good.
If I was you, I'd go through the whole system, including cutting the rotors, turning the drums, replacing the pads and shoes (along with the aforementioned small parts kit, which is only about $7), and flushing a quart of fluid through the system. You could probably do all of that for about $80-100 and an afternoon's time.
