The 2 bodies are vastly different. The 29's firewall is attached to a removable cowl where I believe the 30/31 is attached to the cowl which cannot be removed from the body proper. The widest spot I believe is right behind the doors. I can give you the demension of the 29 at the factory firewall.
Most guys remove the 29 cowl for a gas/fuel roadster. You didn't say what class you are planning for the car