18g is 0.045 and close to original. Much easier to form than 16g (but usually not free). Running boards for sure and maybe firewalls are 16g. The original is actually a bastard size of about a non existant 19g. Some places offer the 3'x3' 20g patch panels which are either "paint grip" or corrosion resistant but I wouldn't use them in any kind of stressed area.
