1972 Duster. Are firewalls different for heater/defroster models versus A/C ?

Stamping changed for '72 &^, looks deeper & there's a big 'eyebrow' arch between the heater hose & blower motor, not sure why. It sucks trying to enlarge an already good-sized hole in sheetmetal, I use industrial conduit knock-out shear punches, used the 4" one to make a strut-cap repair piece for the '96 GC.