The interior is the original, just cut down to allow motor clearance.
I use playing cards for work like this.
I removed the front and rear body sections from the chassis and glued them on the body. I then removed the old posts and sanded the wheel wells from the BACKSIDE.
If you do this, I cannot stress enough how sanding from the backside helps. These bodies are rather thick and you can remove quite a bit to allow a wider stance on the chassis. The Cougar really benefits from the small amount of width gained as it was modeled on the narrow side IMHO.
I like this sheet mount method. On this car the sheet is flush to the bottom. Due to my chassis design, it was clear I needed to adjust the mounts to have the body fit correctly.
So yes, I made an error. But easily corrected.
I mounted a strip of brass over the tops of the original mount tabs. Raised the body just enough to clear.
Chassis in paint shop and almost ready.