So long as your Chevy Silverado has not had any emission system modification/s or if it has, the components which were used are California Emissions approved (they are stamped with an Executive Order Number EO), you'll have no problem obtaining a California smog inspection and do not need to modify anything with your engine's current emission system.
The rule is any USED vehicle which is equipped with the necessary Federal Emission Components is eligible for California registration as long as the vehicle's exhaust emissions fall within California's limits and the vehicle's emission system has not been tampered with.
