More of the seam welding is shown here. Old cars like the Nissan S chassis are not designed as stiff as late model cars so every bit of stiffening helps. Formula D rules do not allow the roll cage to pierce the firewall nor allow for bracing connecting the shock towers to the firewall so tricks like seam welding are used extensively to make things stronger.
The front of the unibody including the core support are totally cut away and replaced with a tubular structure that serves as a front bash bar. Formula D rules allow replacing the front of the unibody anywhere forward of the front most suspension pick up point. The tubular structure is two piece with the lower part serving to protect the front of the car while the upper parts supports the radiator and front fenders. The structure is removable so it can easily be repaired in an accident. This allows quite a bit of damage to be able to be repaired quickly. The front framerail is shortened to the limit of the rules and plated to create an area where everything can bolt too.
You can see the upper part of the structure here. The remote reservoirs for the KW Motorsport 3-way shocks are mounted here. The high and low speed compression damping adjusters are on the reservoirs. For a race car, it is important to be able to reach the adjusters quickly. This really helps in testing.
You can see all of the top tubular support assemblies here. You can see how the bumper, radiator and fenders are hung.
The back of the car has also been replaced by a tubular structure similar to the front. This sort of modular construction makes the car faster to repair and easier to work on. The back of the car contains the fuel cell and drysump tank as well as mounting points for the rear quarter panels and bumper. An easy to reach and sturdy jack point is also incorporated as part of the structure.
The back of the unibody is also plated off and fitted fitted with a square section cross reinforcement. This makes for a strong platform to mount the rear structure to. Impact resistance, repairability and speed of repair are the main reasons for this but having the back of the car so open also makes things a lot easier to work on.