One of the few drawbacks when building a '60s-era GM truck is the limited amount of choices when it comes to updating the rear axle assembly. Since the majority of these trucks came from the factory with manual transmissions, they usually are found to have a rather deep rear gear ratio. Now if you're a stoplight racer or use your truck to haul or tow heavy loads, then a low gear ratio is generally a good thing. Those of us who like to drive our hotrods at highway speed desire a higher ratio to lower the engine RPM. This not only saves wear on your engine, but usually results in better fuel mileage.
Many truck owners change the front suspension from the earlier six-lug to the later '70s style five-lug bolt-pattern disc-brake setup. Although there are six-lug front-brake rotors available in the aftermarket, many rodders choose to swap out the rear axle assembly to a later five-lug model.
Starting in 1960 and running until the 1972 model year, GM utilized a trailing arm/coil spring rear suspension design on the Chevrolet trucks, and a rear leaf spring arrangement on the GMC models. Either brand could be special ordered from the factory with the opposite suspension, which explains why you'll see a Chevy with rear leafs and a GMC with rear coils.
For those truck owners with a coil rear suspension, it's possible to install virtually any rear axle assembly under your truck by changing the mounting saddles. The pre-'70 rearends are 1 1/2 inches narrower than the later '70-87 units.
But as long as a rearend with 3-inch axle tubes (take a look at Ford 9-inch as well) is used, it can be swapped in without much hassle. A new bolt pattern will require new wheels so a change in track width is not a problem as it can be made up in wheel offset.
Replacement axle saddles are available from Early Classic Enterprises that can be welded to a donor rearend at the correct angle and position. The only other dilemma has been the lack of a mounting provision for a Panhard bar on the replacement housing. Early Classic solves this problem with their Super Track Bar kit, which replaces the original Panhard bar design.
Their kit is designed to mount in the original driver's side frame bracket, but instead of the opposite end attaching to the axle housing, it runs all the way over to the passenger side trailing arm. The 12-inch longer bar design reduces the radial arc, which lessens the side to side movement of the rearend as the suspension moves up and down.
Follow along as we show the installation of a later model rearend into a '70 Suburban, along with an ECE 5-inch rear drop kit to change from the original six-lug to newer five-lug bolt pattern wheels.
The coil spring rear
suspension is supported laterally by the Panhard bar, which runs between the driver's-side
framerail and the axle housing. The lack of a Panhard bar mounting point on the axle housing has prevented installing anything but an original rearend in these trucks, without fabricating some type of bracket. Early Classic solves this
dilemma with their Super Track Bar kit, which will replace the original
completely. | 
After disconnecting the driveshaft, parking brake cables and brake hose, the old rearend assembly was hauled out. Shown here is the comparison between the old unit and the newer model five-lug leaf spring setup. | 
The leaf spring pads and shock mounts need to be cut off and the axle tubes ground smooth. A plasma cutter or
oxy-acetelyne torch will make short work of this task, but be careful not to burn through the axle tube. An electric grinder with a flexible wheel was used to smooth out the tubes in preparation for the new mounts. |
Early Classic stamps new rear axle saddles for
the trailing arm suspension, which saves a tremendous amount of time. Although it is possible to cut the original saddles from the old housing, there usually isn't much left when you're through. After the new rearend is installed and centered up, the pinion angle will be determined and the new saddles welded in place. | 
The owner of this Suburban wanted a nice low stance, so an ECE 5-inch drop rear kit was also added. The kit consisted of heavy-duty 5-inch drop coils, KYB shocks, shock mount relocater kit and Super Track Bar kit.
This setup will properly lower and correct the geometry of the rear suspension. | 
Early Classic achieves their lowered stance with computer-designed lowered ride height coil springs that maintain the stock factory characteristics. ECE's Stan Hammond highly recommends using the proper rated spring to ensure that your vehicle's handling and ride are not compromised. |
When dropping the rear suspension, the shock angle changes drastically. In order for the shocks to work properly, ECE designed a new upper and lower mount that stand the shock back up to it's factory angle. This allows the shock to
actuate properly and
dramatically improves the ride. | 
The new rearend was set in place on the trailing arms and located side to side. The new U-bolt slides down through the saddle to attach the rearend housing to the trailing arm, with the Super Track Bar mount captured in between. | 
The new relocated lower shock bracket mounted onto the bottom of the
trailing arm, and was held in place by the U-bolt. |
The Super Track Bar is actually 12-inches longer than the stock bar, and runs from the original driver's side bracket over to the new mount on the right trailing arm. Early Classic uses NASCAR-grade D.O.M. (drawn over mandrel) seamless tubing material, with a TIG welded thread insert for maximum strength. The right side of the bar is adjustable to allow the rear axle to be
centered between the framerails. | 
Probably the most important step in the installation is setting the pinion angle. After
making certain the rear axle
housing was
centered, the
pinion angle was adjusted to match the transmission tailshaft angle using a degree wheel. It is imperative to degree the pinion angle with the chassis at ride height to achieve an accurate dimension. | 
After everything was aligned and the pinion angle set, the new saddles were welded onto the axle tubes. This was a critical step in the procedure as failure to set the pinion angle correctly will result in a vibration and potential driveline failure. If you are unsure of your work, take it to a professional! |
The remaining items left to finish were the brake hose, driveshaft and parking brake cables. A new rear brake hose was bought at the local parts house and installed, and the u-joints mated
exactly to the original rearend. The only dilemma was sorting out the brake cables, but a simple solution was to reuse the original factory units with the new rearend. Ultimately, the intermediate cable can be lengthened or shortened for a custom fit, if needed. | 
With a set of stock ralley wheels and 275-60-15 tires, this project is back on the ground and ready to hit the street. The lowered stance improves both the look and driveability of this Urban Lurkmobile, and the reduced gear ratio is certain to improve the gas mileage. | |