The 350 and 454 are both small-block Chevrolet engines, but they have different displacement. The 454 is larger than the 350, and it produces more power. Because of this, the 454 engine requires beefier motor mount brackets to handle the additional weight and power.
If you are trying to install a 454 engine into a vehicle that originally had a 350 engine, you will need to use the motor mount brackets for a 454. You cannot use the motor mount brackets for a 350, because they will not be strong enough to support the 454 engine.
