If this builds like the 69, there is a small "lip" I believe on the interior bucket that needs to be trimmed.. or firewall... prevents the 2 pieces from joining. Been a while since I built my General Lee, but I remember something in that area.
**edit**
If memory serves... and it probably doesn't... I think "lip" is on the back of the firewall. And that stops the floorboard from setting up high enough.