Jeff Smith: Sounds like you've got the makings of a great machine Michael, it just needs some simple suspension tweaks. You're right that the later-model 12-bolt is 1-inch wider in the rear, with each side adding half an inch. This does not affect handling, but it does change the type of backspacing required to fit wider wheels in the rear wheelwell. With that additional half inch of width, you will need to add an additional half inch or more of backspacing. The idea is to place the wheel where it offers the most clearance. Your Pontiac might be able to accommodate as much as 6 inches of backspacing, such as a 17x911/42-inch wheel with a 275/40ZR17 tire. Taller tires, such as a 60-series on a 15-inch wheel, may not be able to clear the frame as well because of sidewall clearance, but this is a good place to start.
As for the rear suspension components, our experience with A-bodies is that those spindly, stamped-steel lower rear control arms are terrible. Even boxed, they're not much better. If you plan to run a large tire and wheel that will produce less clearance, definitely go with tubular lower control arms, such as the ones from Global West. The Global bar is offered in two configurations. The best unit uses a spherical bearing in the front to allow the bar to articulate, which prevents binding in normal street situations, such as negotiating steep driveways at an angle. Global also offers this same tubular steel arm with polyurethane bushings at a reduced price. Global doesn't recommend poly bushings because the company feels these pieces eventually begin to squeak and can cause problems. For now, leave the upper control arms stock. This is important because under body roll in a corner, Global says the arm must be allowed to twist to prevent binding the rear suspension. This has happened to us, and it causes all sorts of evil handling problems that defy diagnosis.
For the front, we're assuming from your letter that you've swapped to a '70 disc-brake setup. The quickest, inexpensive way to improve handling would be to add a large-diameter front sway bar, such as a 111/416-inch- or 111/48-inch-diameter bar. These bars can be purchased brand-new, but the smart move is to find a front sway bar from a '70 through '81 Camaro or Firebird/Trans Am. These cars used these large front bars, and the cost is minimal. You will have to measure the bars you find, as several sizes were used on second-generation cars; the biggest bars come from '78-'81 Trans Ams. The next step would be stiffer front springs. On our road-course '65 Chevelle, we are currently running 850 lb-in front springs with an aluminum-headed small-block that weighs much less than your Pontiac engine, so this is a good spring rate for your car as well. These springs are also available through Global.
Shocks are another very important consideration. Shock-absorber valving is the main component that dictates ride quality, not necessarily spring rate. Everyone thinks that a very stiff spring will ride like a piece of solid steel between the body and the tires, but shocks play a big part in that as well. A set of stiff shocks with stock springs can ride much worse than stiff springs and properly valved shocks for the street. Ideally, to improve handling, a set of adjustable shocks from Koni would greatly improve ride and handling and allow you to tune each depending upon your requirements. The single-adjustable Koni traditional shocks (PN 8040-1087 front and 8040-1088 rear) are not too expensive at around $215 per pair from koni-na.com. Bilstein also makes shocks for your car (BE3-2972 front and AK-2080 rear) and from Summit these go for $79.95 apiece or roughly $320 for all four. If this is too steep a price, there are brand-new low-pressure hydraulic shocks available from Monroe and Gabriel that are going for as little as around $14 each on eBay.
The biggest hurdle to good handling with stock A-bodies is the front camber curve. As the body rolls, the top of the outside loaded tire (such as the right front on a left turn) will roll with excessive positive camber. This induces massive understeer or "push." The best way to fix this is with a 1-inch-taller '77-'96 B-body spindle and a custom tubular upper control arm from either Global West or Hotchkis. The tubular upper arm from Global, for example, alters the camber curve to roll negative camber, which is exactly what you want to improve handling. If you do this, you'll need to run the lightest rear coil springs you can find. Otherwise, the car will instantly oversteer in the corners very easily, and that can be shocking the first time it happens. You can call Global West for more details on how this works, but we can tell you that our '65 Chevelle track car with gumball 275/40ZR17 Kumho tires can pull more than 1 g on a skidpad with these exact Global West pieces. For a first move, we'd suggest a stiffer front sway bar and better shocks to see if that improves your Pontiac's prowess in the corners.