Well I'm gonna disagree with a bunch of the other people here. First of all, don't get caught up in thinking you have to do too much. Doing too much is the first mistake. Make sure you know what your goals are, because following other peoples goals will lead you astray especially if they are on the racetrack and you are not.
The biggest difference is that on the street, torque is what you feel motivating your butt, while on the track it's peak HP at full throttle that gives the power to beat the other guy all the way down the track. these are related, but very different things. it takes real building to get peak HP, but you can up your torque relatively cheaply and easily for street fun.
The next thing to know is that when it comes to mild hop-ups from a street motor, you'll gain first by increasing the intake side before the exhaust side (you performance guys hold up and hear me out). Why? Because the motor is a big air pump, and it's easier to blow than suck. This is why heads with unequal size valves put the big valves on the intake, not the exhaust side, The motor will blow the exhaust out, but it will feel limitation more on the intake side first. So for a simple hop up this is why it makes more sense to upgrade the carb first before the exhaust. A 4-barrel on stock manifolds makes more sense than headers with a one-barrel carb.
So you're on the right track with the four-barrel intake. Sure, your potential will be limited after that by the exhaust but that's ok.
So next, what carb? Keep it simple and small, a nice old squarebore carter like an AFB or AVS will work nicely on your squarebore intake manifold. You don't want a big modern spreadbore carb, it's too much. Keeping it down to 500 or 600 cfm is plenty on your 225.
And contrary to what some have said, I find a 4 barrel to be not only better performing but better fuel mileage than a 2-barrel! Why? Well, the primary bores of a four-barrel by themselves are more economical than the one-size-fits-all- bores of a one-barrel or 2barrel. Sure, if you put your foot down you can also burn more gas. But don't assume that a four barrel will necessarily hurt your mileage. You'll be surprised at how much smoother it is, too. (those who say it will do nothing - I bet they haven't swapped out a one-barrel lately)
Now in order to install that four barrel you're going to have to change the gas pedal and linkage on your car. look on ebay for a 'v8 gas pedal' for an a-body. What that really means is a non-one-barrel pedal, which has a cable linkage instead of the weird torsion rod linkage that slant six one-barrels have. But that's an easy bolt-on change, not too expensive.
So with that upgrade you will definitely increase your 'fun' quotient! And you could take it one step further by installing slant six headers.
But at that point I would suggest you stop. Why? Because anything more than that and you're really getting into engine building, machine work, and a much more substantial budget. And at that point, when you spend that coin, the slant six will give you less bang for your budget than a V8.
Much better is to increase your fun by upgrading the brakes and suspension - at very least swap the 9" drums, if you have them, to 10", which are at least enough. And even better, swap up to later disk brakes. New torsion bar front springs are an inexpensive way to massively improve handling, and if you get into it you might even consider a sway bar.
PS - final word of advice, if you're in high school: never sell your car. When you're 50 it will be worth so much to you to still have it, and then you'll have the budget to turn it into the race car of your dreams. Until then do some upgrades, warm it up, enjoy it, and don't let it turn into a project that stays off the road for very long!
