Sheet metal question

[jamdcm]

Hey guys, I have a question about sheet metal repair. I have a 56 /57 GMC/Chevrolet (56 chassis 57 big window cab) that is in need of repair. I’m also going to combine another cab to make it an extended cab. My question is do I have to use the same thickness metal for body repairs that can only be seen from one side. I’d like to smooth over the firewall shave door handles and make filler panels for the extended part on the side.

Any advice would be greatly appreciated