When installing a SBC GEN 1, these are all you need for engine mounts. If you are doing an LS or LT GEN V conversion, you need the conversion mounts that bolt to the block to accept the SBC mounts. 

You will have to remove two rivets per side on the inside of your front cross member.

Place the side mounts on the frame as shown and make sure the tubes on the new side mounts are parallel to each other.

Mark the frame. Remove and drill new holes on top of the frame.

Install side mounts onto frame using supplied 3/8 bolts, washers, and lock nuts.