When it comes to handling, we need to take a look at the rear springs and the hangers as well. Chrysler engineering hit a home run when they designed the leaf springs for muscle-era Mopars. The biased design with an extra leaf on the passenger side helped control the axle on high-powered cars, as did the use of a shorter front segment (distance from the axle to the front hanger). These two design features made the Mopar rear suspension the best in the business for many years. Even today, a lot of cars are still racing every weekend with original style rear suspensions under them.
Torsion bars are available...
Torsion bars are available in a wide range of sizes. Factory bars were color coded for size, but paint markings are usually long gone, so carry calipers and a tape measure if buying used bars.
Mopar Performance offers a few different leaf spring packages including the XHD springs for street performance, Oval Track springs for track days, and Super Stock springs for drag racing. Various aftermarket vendors also offer different leaf spring packages for the A-, B-, and E-Body cars. The factory XHD leaf spring package is a good compromise for daily drivers. It has a reasonable spring rate that won't beat you up, but has enough strength to handle a healthy big-block engine. Drag racing puts a huge amount of stress on the rear suspension, which is why the factory designed the original Super Stock spring package. While the Super Stock springs work great on the dragstrip, their heavily biased design isn't recommended for cornering/handling, and the high spring rate can create a fairly harsh ride on the street. Some people have had success using these springs in road race applications by using two left side Super Stock springs, so that is an option for ambitious car crafters.
The use of different spring packages often requires the use of different spring hangers as well. There are a lot of different combinations available, so talk to the vendor to make sure that you have everything required to swap in the new parts. The installation of Super Stock springs can become fairly confusing on some cars due to the various spring lengths used by the factory. When in doubt, use your tape measure and study the specifications before you order the parts. For example, early B-Body cars are especially confusing due to their 21-inch front segment springs. In most cases, upgrading the rear springs on these early B-Body cars works best if you use A-Body springs in conjunction with special spring hangers. The Mopar Performance catalog isn't clear on this, so you need to do a little research before ordering parts. Mancini Racing can help you get the correct springs and spring hangers when looking for the Super Stock stuff.
Leaf springs come in several different varieties. There are monoleaf springs, (one plate of spring steel), and these are usually thick in the middle and get thinner toward the ends. Multi-leaf springs consist of several leaf springs of varying lengths, stacked on top of each other. The actual spring rate will set the tone for the rest of the suspension setup. This means how fast the car reacts to driver input (throttle, brake, turn in, etc). Spring rate dictates how stiff the car's ride is, and how much squat it will experience under hard acceleration.
A variety of leaf springs...
A variety of leaf springs are available for A-, B-, and E-Body cars. These Oval Track springs from Mopar Performance have an odd 20-1/2-inch front segment, which moves the tire slightly rearward, so they require special hangers. XHD (Extra Heavy Duty) springs are a bolt in for most cars, while Super Stock springs usually require a longer spring hanger.
If you're looking to dramatically improve the cornering performance, traction, and response of your Mopar, and want a bolt-in kit, Hotchkis Sport Suspension makes what they call a geometry correcting leaf spring. According to them, it is specifically designed to reduce roll steer. The kit features a bolt-in relocation bracket for the front of the spring, paired up with 160-lb/in leaf springs to improve handling performance and balance. Keep in mind that this spring is designed to lower the car approximately 1-inch. The lowered design reduces the vehicle's center of gravity.
Other than buying new rear springs, what can be done to improve the handling capabilities of a rear leaf-spring suspension? Controlling the lateral movement of the rear end under hard cornering can be accomplished by installing a Panhard rod or Watt's link. If you're not looking for something that extreme, you can also spend a lot less and get good results by swapping your OE bushings for better performing polyurethane pieces.
Adjusting the ride height is not nearly as simple with leaf springs as it is with the torsion bars. To help make it simple, you can get the Hotchkis leaf springs with their 1-inch drop, or Mancini Racing offers leaf spring hangers with a second eye bolt location to lower the car. Heating the springs to lower the car is not recommended. The use of a spacer placed between the leaf spring and the axle to lower the car is generally not a good idea on a race car, although a thin spacer might be OK on a street cruiser.