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This page summarizes some of what I've learned running my coupe in the class over the last couple of years. Bear in mind there are lots of modifications out there you can make, what I'm trying for is to cover the ones that gave me the most bang for the buck. The car in question here is a '65 Corsa 140/4, class is E/SP. Street Prepared allows any part from any Corvair or Stinger (stages I, II, and III) to be used. Engines, transmissions, and differentials must be swapped as a unit, but any other parts may be freely interchanged. Additionally, most bolt on engine and suspension parts, like headers, ignitions, sway bars, and springs, can be replaced or modified in any way.
First a few assumptions - The car must be in good mechanical shape. An autocross isn't that hard on the car, but any flaws will quickly be made very obvious. There is also a safety check, now is a good time to look over the brakes, tightness of the ball joints, steering box, etc. Fix any problems you find first. Next, we are going to be running in several events per year, and are making a real attempt to be competitive.
This point is rather more important than most people think. Successfully competing in anything will require some amount of dedication. I don't say this to scare anyone off, I just don't want people to think that everyone there is just showing up and running. It took me well over a year of regular events to get my coupe to be halfway fast. Don't forget that this type of driving will require some practice as well.
Ok, enough caveats and junk, on to the fun. IMHO, the single most important thing that affects an autocross car's performance are the tires. There is considerable debate over which are best, but in the beginning it doesn't really matter, there are two major reasons why we are doing this, and neither is that we're looking for that last 1/10 of a second:
1 - Regular autocrossing (say 10 events) will completely wear out your street tires. It is cheaper to run real tires than to "save money" by trying to get away with a single set of tires. Sometimes you can get closeouts of the previous year's tires, or buy a set used if you're on a tight budget.
2 - R-compound tires have so much more adhesion than even high performance "normal" tires that tuning the chassis and learning to drive on anything else is, to a degree, a waste of time. The car rolls more, the bushings deflect differently, and the tires break away differently.
You'll need another set of wheels to mount your tires on. Fortunately, late model Corvairs use the standard 5-lug Chevy pattern, so if you're on a budget you can probably score a set of wheels for $100 or less. You want some that are at least 7" wide, shorter is better. Try for a 215 or 225 section width tire. A note here - although 13x7 wheels would be the best selection, they are not very common. 14x7 is a pretty common size for lots of Camaros, and 15x7 was standard on many 82-92 Camaros and Firebirds, finding them is usually easy. Take a look at my tire article for more info on sizing wheels and tires.
The next big problem you will likely face is the dread carb cut out problems. In a nutshell, cornering makes one side starve, and often the other side floods. Re-locating the jets, either by rotating the carbs, or moving the jet inside the bowl, will cure the starving. Installation of vent tubes, along with plugging the hot soak vent (the one that goes down to the carb base) will solve the flooding. You can also use one of the various setups to adapt a Holley or Weber carb onto the engine, but be aware that milling off the intake is a no-no, so since you're stuck with the stock intake, you won't gain very much for the expense.
At this point, you'll want to start considering a few other mods. These are in no particular order, as I'm not sure which ones will be the most effective for you, and you should consider the difficulty of installation too. A much longer list of SP legal mods is here.
Note that other than the carbs, there are no engine mods listed. That's because autocrossing is not biased that much toward power. Yes, a car with more useable power will be faster, but a car with a properly set up suspension and a pretty stock but healthy engine will eat a car with a killer 200 HP engine and no way to get through a corner. Autocross straightaways are very short, and you are competing against guys running V8's. It is highly unlikely you'll do well by trying to play their game and win with horsepower, but you can do well by playing to a Corvair's strengths - brakes and suspension.
Of course, sooner or later you're going to want more power anyway :-) Once you are really happy with the handling, then it's time to worry about the engine. If you're planning on re-building, remember that the engine must be to either Chevy specs, or to a stage of the Stinger specs internally, you may not mix and match parts. I'm putting together some comments on some of the more common engine modifications.
At this point, you should have a fairly well set up, fast car. You should also consider taking one of the SCCA autocross classes (there is usually one offered at each of the National events). Now it's time to look for any other modifications that are can be made. I've run through the list of allowable mods and detailed some stuff to think about on this page.