68 SWB - Lets start with the easy answer. There are several good quality kits on the market. Hotchkis, Ridetech, Porterbuilt, Early Classics, and others. You can spend $4000 to $6000 and get it done. And probably, the next guy will run faster than you. Does that mean these parts are no good? No, they are good parts, but thats not enough. It's the other things that will make the difference. I like 18's better than 20's, but thats me, and unsprung wieght makes a difference on the track, and the ride is better. Tires win races. Kumho, Bridgestone, BFG, Falken, but stay with 300 treadware or less. Most Autocross clubs, and Goodguys have a 180 treadware rule. Wieght is a big issue, put that pig on a diet, and focus on the front end. Every pound helps. Motor placement, If you can move the motor down and back, DO IT!!
OK, so on to your question. Spindles and springs, I like McGauphy's or Western chassis, spindles. Doetch tech lowering springs are pretty good, so are Hotchkis. If your Really cool, you'll learn how to tune springs, and start searching some Oval track suppliers, who have many more choices avail. I have used Porter, Ridetech and E.C. arms, and like them. I have not used the new Hotchkis arms yet. If I didn't mention others - well, you guess. In the back, 2" block and 4" spring. I like the E.C. raised crossmember, but others are good. Swaybars. GO BIG, but keep the back within 1/4" of the front. Move the tank back. I sell them, buy one so I can eat. Move the battery back. I sell battery boxes too
. Take the time to shim the pinion angle. It's free HP. Here's what you forgot. BRAKES. Save up and do it right, and put an adjustable proportioning valve in the rear line. Steering, AGR or LEE box. If you get out to the 7-9 deg caster range, you will need to work on bumpsteer. Hotchkis's center link will help, or modify the stocker. Replace the rag joint and coupler with good quality U-Joints. Again, I sell the Flaming River joints. Steering feedback is needed. Spend $ on shocks. Now it's on you. Let us know your plan, and the results.