If we're talking about the car in your avatar, the grilles make it a '67 or later. By then the number had gone from the alloy plate on the MC box and was indeed stamped into the bulkhead above the heater box. I've not seen it anywhere else. Some heavy primer and a couple of coats of paint will fill in the numbers. You probably don't want to sand it down across there but that'll be the only way to get the number. If you get the light right you might just see an impression of it. The number on the plate on the inner wing is the factory body number. It will be close but likely between 10 and 30 different to your chassis number.