I should have also linked you to a company whose products I have used, Alumilite.
One of their tutorials applies particularly well to your application: Making a Slot Car mold (which also links to the procedure for casting a slot car body from the resulting mold). There's also a tutorial on Squish Molding, and Molding a Taillight.
In the second tutorial, the vent holes that allow excess resin to rise out of the upper mold half (which would be the carbody-facing side of the casting in your case) as it's pressed into the lower could be the rear-ward extensions that the chassis screws go through.
By the way, do you have hi-res versions of the HoD pantograph photos? (particularly slightly overhead side views and closeups of the arm assemblies) If Duddy doesn't have anymore, I may need photo references to scratchbuild my own, most likely from brass rod/tubing/sheet. My email is in my profile.