Remove the large nuts that secure the mounts to the engine as well as the two (on each mount) that secure the mount to the frame post. Then, lift the engine just enough to slide the old mounts out. When you install the new mounts, loosely install the top nut while the engine is still lifted and then slowly set the engine back in place. Make sure the lower mount and frame holes line up and drop the bolts in before completely setting the engine back in place. Tighten all nuts and bolts.
You can lift the engine using a floor jack and block of wood. Place the block of wood at the front of the engine, either under the harmonic balancer or at where the front of the oil pan is bolted to the engine. It might be necessary to remove the lower radiator hose to get the engine to lift high enough.
