The failed smog test report will contain valuable information about what caused the car to fail. If it was excessive hydrocarbons; the problem is usually that the ignition timing is set too far advanced, or that the EGR valve is not working properly, the spark plugs or plug wires need replacement, or that there is low compression in one or more cylinders. Some engines won't pass smog when the timing is set to factory specs, and have to be retarded beyond factory settings to pass. The 1992 Toyota V-6 engine was a flawed design, and your engine may have compression problems as a result. Toyota replaced many of those engines under warranty years ago. But that is no longer covered.
A smog test failure from excessive carbon monoxide indicates a problem in the fuel system; often a bad oxygen sensor, dirty fuel injectors, leaks in vacuum hoses or hoses connected improperly, bad mass airflow sensor, EGR valve inoperative, or throttle position sensor defective or out of adjustment.
The mechanic probably replaced the air duct because the original one was missing or damaged. The head gasket set sounds like there was a coolant or compression leak. Both of these items are real and necessary. But the question now is how much additional work will be needed to make the truck pass smog. Here in California, there is a law that places a limit on the cost of repairs required to make a vehicle pass smog. If the cost of repairs exceeds a certain amount; you may be eligible for financial assistance from the state; or you may have to take the vehicle to a smog referee for a determination or further action. Contact your state bureau of automotive repair for more details.