[munter]

As the cars you mention are probably mostly quite narrow I suggest you investigate adding weight low and as central as possible to the chassis.
This of course  means opening the car up and adding weight(material of your choice) to the chassis where possible that doesn't interfere with the interiors or other parts.

Also when refixing the body and chassis dont crank the body screws up really tightly....allow a little movement between the two parts.