We began our conversion by removing the stock 7.5-inch rearend housing out of our '67 Mustang so we could accurately measure the position of the spring mounts.
Car crafting is all about building a strong, reliable performance car from readily available and inexpensive parts. It's also about not spending the next year's worth of paychecks in the process. Right about here is where we're going to piss off a bunch of hard-core Ford guys. The standard Blue Oval rearend rule for the last 40 years has been "Use a 9-inch." While we will not dispute the strength of a nodular iron 9-inch, the more recent Ford 8.8 is a realistic alternative.
The only true way to discover if the 8.8 in question has a Traction-Lok limited slip is to remove the rear cover. If the differential has that S-shaped spring (arrow) in between the side gears, you've scored.
The focus of this buildup will be an 8.8-inch housing we pulled out of a wrecked '88 Mustang and rebuilt to spin the wheels on our '67 Mustang future road racer. With assistance from our buddy Tim Moore, we figured out a way to stuff this 8.8 under the early Mustang while spending less than half the cost of a typical $1,900 9-inch Ford. The end result is a rear axle assembly that's lighter and more compact and will be just as durable behind our small-block as any 9-inch-while not permanently maiming our wallet.
Boneyard Beginnings
It's no secret that the '80s Fox-body 5.0 Mustang can be found in virtually any boneyard in the country. All 5.0 'Stangs came with the 8.8 rearend. As we outlined in our original 8.8 buildup story in the June issue, "The Ford 8.8 Mustang Rearend," this third member can be purchased for much less than $100 and, if you're lucky enough to find it, a housing with decent gears and a Traction-Lok limited slip. The rearend is so designated because of its 8.8-inch ring-gear diameter, making it slightly larger than the much-vaunted GM 12-bolt. If there is a weak link in the 8.8 chain, it's that most Mustangs came with spindly 28-spline, four-lug axles. But there's an easy fix for that too.
We used the old 7.5-inch rearend out of our '67 Mustang as the template for the new 8.8. First we measured the distance between the centerlines of the original spring pads, which was 4231/44 inches.
Our automatic-equipped donor car was handicapped with a lame 2.73:1 gear ratio, but at least it came with a factoryTraction-Lok limited slip. Other vehicles equipped with the 8.8 rear can offer a wide range of ratio opportunities, so it's possible to find 3.45:1, 3.55:1, 3.73:1 or even 4.10:1 factory gears if you're lucky. Unlike the GM rear axlehousings, there is only one style of ring-gear carrier for all gear ratios, so if you find an 8.8-inch Traction-Lok, it will work regardless of the ratio all the way up to 4.56:1.
The quick fix to convert the Mustang 8.8 over to five-lug bolt pattern axles is to find a pair of left-side '90 and newer Ford Ranger 8.8 truck axles. The left-side axles are the same length as the Mustang pieces and will bolt right in. Of course you'll also need the drums, which will fit right over the existing Mustang backing plates and brakes. The smart move would be to just drop a few more bucks and buy new drums, which is what we did.