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Except for some early firewalls produced before the release of the engine vibration dampner, all subsequent firewalls were produced with those four holes, regardless of the powertrain, chassis type or body type destination of the firewall.
The only surefire way to determine the truck's original powertrain is the engine number stamped on top of the chassis frame rail near the left foot of the firewall. If that number has a 18 or 18F prefix, it was produced as a V8. If that number has an AB, B, ABF, or BF prefix, it was produced as a four (the F being for those engine/transmission assemblies destined for RHD chassis).