It’s always important to do a “cross out” measurement to make sure the unibody is square. This involves choosing four equal reference points (body mount bolts suffice if you cannot locate the original “gauge holes”). Using a tram gauge or plumb bobs, measure diagonally, and compare the two measurements. As long as the reference points are equal, the measurements should be very close. Factory tolerance is 1/8 inch, so if the numbers are within 1/4 inch, consider yourself lucky!
The strength of the body-on-frame design is undoubtedly the highlight of this design but many applications still require additional bracing to keep the structure solid and stiff. Early Chevy Impalas (1958–1964), for instance, feature an X-frame design, as opposed to the more generic perimeter design of many other makes and models. Don’t ask why General Motors thought this was a good idea but the X-frame certainly doesn’t lend itself well to high-performance driving.
Moving on through the years, General Motors stepped up its game with the A-Body platform but it took the perimeter design a bit too far by building a strong structure around the perimeter of the car. This leaves a lot to be desired in the crossmember and bracing department, making for a frame that flexes during harsh driving conditions. For most full-frame cars (especially the GM A-Bodies) additional chassis braces are necessary. Luckily, they are widely available to fit the popular 1964–1972 GM midsize models.
In most cases you can spot a full-frame car just by the size of it. Ford and General Motors placed full-size models (Impala, Bonneville, Galaxie, and Monterey, among others) on a full frame, based on the weight of the vehicle. They felt the heavier car needed a full frame to support it, and they were right.
If rust or chassis damage is an issue for your project, you can always opt for aftermarket components to repair the damaged metal. Of course, this requires some intense fabrication and welding skills, depending on the severity of the damage. For chassis and floor repairs, a MIG welder is preferred for its ease of use. Starting with a rust-free car is ideal.
When General Motors introduced the A-Body platform in 1964, it was the backbone of the first string of muscle cars. The Chevelle, GTO, 442, and GS shared the same chassis, which featured a multi-link rear suspension and a perimeter-style frame. This design leaves a lot to be desired because of a lack of bracing around the suspension mounting points but it’s a strong platform to build upon.
Corvettes can sometimes be confusing to the novice because they were not technically a muscle car or a pony car. They were considered a sports car, and they featured a body-on-frame design from day one. Corvettes also featured a fiberglass body from day one, and an independent rear suspension starting in 1963.
Chrysler Corporation didn’t play into this mindset, choosing unibody construction for most of its models, starting in 1960. The heavy weight, along with metal deterioration caused by rust, spelled disaster for many Mopars from the 1960s and 1970s, which is part of the reason they’re worth so much money. There aren’t many of them left!
Despite the original flaws and deterioration after years of use, full-frame vehicles provide the necessary strength for a bulletproof Pro Touring build. You may need to strengthen a few points on the frame, especially the areas around the suspension mounting points, but you’ll save quite a bit of time and money on things such as subframe connectors, shock tower braces, etc., on a full-frame design.
The weakest points in a full frame are usually located around the “kick up” where the frame rails go over the rear-end housing. These joints can sometimes flex, and they are susceptible to rust, which causes the joints to weaken even further. For the suspension to do its job, the mounting points must be capable of taking the stress of harsh driving without flexing, as even 1/16 inch of flex can cause changes in the geometry. Unfortunately, full-frame cars are not the norm in the Pro Touring world, so most folks have to spend a lot of time and money to make their unibody vehicle as sturdy as a body-on-frame vehicle.
With this in mind, a number of enthusiasts have built custom frames for cars that originally used unibody construction. This is a major task, as it usually requires the removal of all floorpans and trunk pans to even get started on a custom chassis. Then new pans must be fabricated to work with the chassis, and this usually leads to the original result of full-frame cars: extra weight. It certainly isn’t a beginner’s project. The end result is usually a full-on race car, instead of a practical Pro Touring vehicle that you can legitimately drive on the street.
Until the 1960s almost all American auto manufacturers used the body-on-frame configuration, even though Nash introduced the first successful unibody car in 1941 after a somewhat flawed attempt by Chrysler almost a decade earlier. The term “unibody” is actually short for unit body, which actually means unitary construction. This type of construction means that the body and chassis are essentially the same unit.
With a unibody design, the floorpans, rocker panels, and inner fenders are part of the vehicle’s chassis structure. With most muscle car and pony car applications tipping the scales at 3,000 pounds or more, that is a lot of weight to be supported by sheet metal. The advantage of unibody construction for auto manufacturers was weight savings and ease of construction at the assembly plant. Generally, the car would technically have frame rails on the front and rear of the vehicle but these rails would be welded directly to the floorpan and cowl area. General Motors was the exception with its bolt-on front subframes.
This unfinished Camaro provides a great visual of a unibody chassis construction. The satin parts are essentially just sheet metal, used for the floorpans, trunk pans, etc.; the glossy portions are structural components. This example has been modified with subframe connectors and other structural bracing.
General Motors is the only manufacturer that used a bolt-in front subframe. The frame unbolts from the chassis, unlike the Ford and Chrysler unibodies that feature a welded front frame section. This removable subframe makes for easy assembly but suffers from a bit more flex due to the movement of the bolts and bushings.
It wasn’t until the late 1950s that Ford adopted this chassis design for its larger cars, such as the Lincoln and Ford Thunderbird. From there the trend grew, and Ford used a unibody construction on other models, including the Falcon, Mustang, and Torino (which was changed to body-on-frame in 1972). Ford passenger cars have always been plagued with minimal engine bay real estate, thanks to huge shock towers. It is a major chore to stuff a large engine into any early Mustang, Falcon, or Fairlane without doing major work to the front suspension.
Detroit Speed’s new Aluma-Frame for 1964–1970 Mustangs is a bolt-in front suspension module that replaces the original front suspension and removes the shock towers altogether. The Aluma-Frame retains the original front subframe rails and provides lots of great advantages, including improved suspension geometry, rack-and-pinion steering, and adequate engine bay clearance for many modern power plants. (Read more about this front suspension system and how it helps Mustang