Only real difference is the pan. 04 has a steel oil pan that has a separate bracket that bolts to the block for the motor mount. 06 has an aluminum pan with an integral boss for the mount. Fitment is the same as to going in the car. In fact I have an 06 l26 topswap in my 97 Monte Carlo. Seemed like the 06 pan has a bit more ground clearance. As for the rest of the motor its exactly the same