Axle Exchange
Out back, the Fifth was equipped with a puny 7 1/4-inch rearend, like many of these cars, and fitted with incredibly high 2.2:1 gears. Obviously the 7 1/4-inch wasn't up to the task of handling the 400 ft-lbs on tap, so a rearend swap was part of the plan. The largest production rear axle used in the F/J/M-Body chassis was the 8 1/4-inch, not as tough as the legendary 8 3/4-inch, but much more durable than it is given credit for. Since it would be a simple bolt in, and these axles are common as dirt, an 8 1/4-inch rear got the nod. This wasn't going to be a drag car, but with the power from the hot 360 on hand, the Fifth would make a superb high-speed cruiser. I went looking for an 8 1/4-inch rear with a 2.71:1 ratio, and found it in a Diplomat police package car that would be a source for other things as well.

The Cop Connection
One of the nice things about building the M-Body is that the wreckers are flush with cars ready to donate parts to the project at rock bottom prices. Throughout the '80s, the M-Body Mopars were by far the most popular police vehicles in the nation. At least out west, there's a glut of junked police package cars with a wealth of special and heavy duty components ripe for the picking. No, high-performance engines aren't part of the bounty, since all of these cars were equipped with 318s, but the chassis components fit any regular F-, M-, or J-Body car and make them stick like glue.

The Fifth now had the stout 360, a beefy modified high performance 727, and an 8 1/4-inch that was on its way out of a derelict police cruiser. Why stop there? The police car's rear was hanging on a set of special H/D police springs, and located to the chassis by a stout sway bar. The whole enchilada, including the rear axle assembly, brakes, springs, and sway bar was pulled as a unit, upping the ante on the rear of the Fifth to a full police package suspension. The assembly just rolled under the Fifth and bolted-in; the Fifth's frame was even already drilled out for the sway bar brackets. A set of new KYB gas shocks from Performance Suspension Technologies finished off the rear. With the trans and rear-axle change, the driveshaft would need to be about 4 1/2-inches shorter than the original. The stock shaft could have had the trans yoke changed to the larger-splined 727 piece and shortened, but instead a shaft was simply culled from a junked 727, 8 1/4-inch equipped police car.

With the rear now sporting the police hardware, the wallowing front suspension of the Fifth was next. Police package cars came with special H/D torsion bars, and a massive 1.125-inch front sway bar. The transverse torsion bars are much easier to remove than the traditional longitudinal bars of older Mopars. The T-bars and sway bar from the front of the cop car were extracted in an hour and a half at the boneyard with basic hand tools. The service manual details on the procedure, which we will not repeat here (if you are going to do this much work, you'll want a copy anyhow). As in the rear, the cop stuff was just bolted in, and the front suspension got a fresh set of KYB shocks from PST as well.