The engine mounts for the 2004 5.3L engine will not match up with the frame mounts of the 1985 GMC truck. Custom engine mounts will need to be fabricated or purchased from a third-party supplier.
Transmission
The 700R4 transmission in the 1985 GMC truck will not be able to handle the power and torque of the 2004 5.3L engine. A stronger transmission, such as a 4L80E, will need to be installed.
Drive Shaft
The drive shaft from the 1985 GMC truck will not be the correct length for the 2004 5.3L engine and the 4L80E transmission. A custom drive shaft will need to be fabricated.
Exhaust System
The exhaust system from the 1985 GMC truck will not be compatible with the 2004 5.3L engine. A new exhaust system will need to be fabricated or purchased from a third-party supplier.
Electrical System
The electrical system of the 1985 GMC truck will need to be modified to work with the 2004 5.3L engine. This may include installing a new wiring harness, computer, and sensors.
Cooling System
The cooling system of the 1985 GMC truck will need to be modified to handle the increased heat output of the 2004 5.3L engine. This may include installing a larger radiator, water pump, and thermostat.
Fuel System
The fuel system of the 1985 GMC truck will need to be modified to supply the 2004 5.3L engine with the proper amount of fuel. This may include installing a new fuel pump, fuel lines, and injectors.
Brakes
The brakes of the 1985 GMC truck may not be adequate to stop the vehicle with the added weight and power of the 2004 5.3L engine. Upgraded brakes, such as larger rotors and calipers, may need to be installed.
Suspension
The suspension of the 1985 GMC truck may not be able to handle the added weight of the 2004 5.3L engine. Upgraded springs and shocks may need to be installed.
