Here's an idea for ensuring symmetry of both sides of the vehicle. Build a mirror image pantograph and pick one side to be the master while the other gets carved.
How to build one? Not a clue!
Look up this US patent. 6789985
Such a gizmo would be a big help to doing body repair, especially when re-shaping non-removable panels. The good side would be the master while the filler on the crunched side would be carved into a mirror copy.
On your foam car core you'd find out which spots on the carving side are low compared to the master side.