Installing Stifflers’ FIT System for Improved Chassis Rigidity

The roots of our beloved project 1997 Mustang GT’s chassis trace back to the 1978 Ford Fairmont. While state-of-the-art in the late ‘70s, the Fox platform was never originally intended to accommodate convertible models. Ford breathed new life into the Fox platform with an update for the 1994 model year. Dubbed SN95, The Mustang’s fourth generation included chassis provisions for convertibles. However, the flexible roots of the Fox platform still underpinned Mustangs until 2004.

Like most unibody cars, the Mustang’s roof is a major structural element of the chassis. So, when it’s chopped off to produce a convertible, chassis rigidity will inevitably suffer. One way to regain some rigidity is to fortify the floor pan. Stifflers in Plainfield, Indiana, offers their FIT (Fully Integrated Technology) System that includes subframe connectors, stiffening rails, and a web brace to tie it all together. The result is a stiffer floorpan that drivers can immediately feel while driving over rough surfaces. Stifflers’ FIT System is a must-have for any 1979-2004 Mustang convertible.

The Stifflers FIT System is constructed of mild steel tubing that’s finished in a durable powder coat finish, with masked-off weld areas to speed up installation. The Slide-2-Size web brace accommodates various installations to ease welding and add rigidity.

The subframe connectors mount entirely under the floor, so there’s no floor pan cutting or interior gutting required. The stiffening rails provide a convenient way to jack up your Mustang, and Stifflers claims the FIT System adds additional side impact protection.

To try it for ourselves, we installed Stifflers’ FIT System on our project 1997 GT convertible. To see how we did it, check out the following photos and captions.

1.The Mustang’s roof is a major structural element of the chassis. Convertibles don’t have one. Stifflers in Plainfield, Indiana, offers their FIT (Fully Integrated Technology) that includes subframe connectors, stiffening rails, and a web brace that ties it all together and stiffens the floor pan.

2.The Stifflers FIT System is constructed of mild steel tubing that’s finished in a durable powder coat finish, with masked-off weld areas to speed up installation. The Slide-2-Size web brace accommodates various installations to ease welding and add rigidity.

3.Starting with the subframe connectors, we held them up against the floor pan to plan where we wanted to weld.

4.Then, we prepped the sections for welding by grinding away the powder coat in a few more places just for good measure.

5.Then, we marked the floor pan and sanded away the paint in locations that corresponded with the bare sections on the subframe connectors.

6.With the subframe connectors tight against the floor pan (and the car’s battery disconnected), we fired up the welder and got busy.

7.Next, we fit the stiffening rails against the rocker panel pinch weld, and ground away the paint using the same procedure as the subframe connectors.

8.Then, we welded the stiffening rails to the rocker panel pinch weld

9.We used a block of wood to hold the rocker panel covers away from the areas we wanted to weld.

10.With the subframe connectors and stiffening rails welded into place, we moved on to the web brace.

11.We fit the web brace between the subframe connectors and the stiffening rails, ground away the powder coat, and welded them up.

12.Once the welds cooled, we painted the exposed metal and welds with satin black paint. It matched the powder coat pretty well.

13.You can see how the Stifflers FIT System adds rigidity to the floor pan, and how the rockers are more resistant to side impacts.

14.The Stifflers FIT System made our Mustang feel more solid and planted over rough pavement and in hard corners. We expect fewer rattles and wiggles as we rack up the miles on this drop-top GT.