You're welcome to check out my webpages to see what I and Dave Mapes have done. Both of our websites are a bit dated, but we're probably the most active SCCA autocrossers on the east coast in a-bodies.
A lot of your success in class will depend on your local competition. ESP will potentially put you against some pretty well developed later GM suspensions. But the game is heavily dependent on the driver - and the drivers ability with car they are driving.
I've actually had my some of my best success on the small tight lot at Boeing. That's proof that a portion of it is mental because I've been running in Street Mod which had all sorts of cars large and small.
Right now its hard to know where any of the classes are going as they attempt to completely rework 'stock' into 'street'. SP is a reasonably place to start, but also see if a local 'street tire class' is an option. If you're just running for fun, C Prepared is always a safe bet. You'll just be very lightly 'Prepared' relative to the rules.
OK. My opinion on prep for you and your car:
Build aerobics, arm strength and hydrate smart.
*Built 318
In SM or Prepared it is whatever you want, in SP this is a stock 273/318/340 with whatever intake, carb and headers you want. I'm sure its simply from a cop car
*904 auto, column shifted
Might be an issue with the front pump after some abuse. e-mail or PM because I don't recall the details. Mild shift kit like Turbo-Action's is useful in reducing wear and crisping up the shifts on courses that requre 2nd. I do not like to manually shift on course - especially with the column shifter.
*3.23 rear gear
Good enough unless this is a dedicated competition car. Every course is different.
*8 3/4 B-body rear axle, NOT narrowed.
Probably OK, just watch the tire clearance to fenders
*10" discs front, 10x2.5" drums out back, non-power
Good base. Fine tune with choice of lining materials, wheel cylinders and prop valve if needed.
*14" steel wheels wearing 215/75s
OK for your first play day or two as this will teach you patience and smoothness! If these are nice new tires with big blocks then maybe a set of used 215/70s will save those new tires for what you intended them for. A used set of aluminum rims in 14x6 minimum and 15x7 preferably will be useful for mounting some fun tires on. Steel starts getting heavy as the wheels get bigger, but tire choices get better. Tires get heavier too (and the investment gets high so I don't push anyone to start there)
*Factory 26:1 manual steering
Might be the one item that will frustrate you when autocrossing
*Rebuilt front suspension, urethane bushings
OK
*Magnumforce upper control arms, aligned with 5.5 degrees of positive caster!
This might snap that steering wheel back in your hand! I don't think this is a great match to the tires, but with sitckier tire will OK
*1" t-bars (worth every penny btw)
Good.
*New heavy duty leaf springs out back, stock height
If it wraps up or hops with sticky tires add more clamps to the front half of the spring.
* NO sway bars installed
Second most likely item that might frustrate you. Get a front sway bar. Size depends a bit on what tire you will be using. Stickier tire = more grip = more roll and therefore more roll resistance will be wanted. That said, anything is better than nothing unless you are rock crawling or mudding. So get something, but just up front to start with.
*KYB shocks up front, parts store cheapies out back
Gotta start somewhere. If you get serious you'll replace these and this is another area that can get very expensive, so don't rush in here.
*Stock interior, bench seats, no seatbelts (yet)
As stated by others - good lap belts are a must. Shoulder belt - some don't even like 'em for autocrossing. A good bucket is help, but I've autocrossed on benches. Wedge your left foot in the corner and butt in the seat. See number one suggestion. 
The slow steering hurts most in the slaloms. In all cases start turning the wheel early and look ahead, not at the cone your passing but where you want the car to go. If the front wheels don't bite, its because you were braking too late and too hard. Slow down and they will miraculouly turn the car. If you get out of shape or off line, slow down and get back on line. This is the hard stuff; the self discipline.
