In order to determine which set you need you will simply measure your frame side bracket. 2-3/8 Inch - If you measure 2-3/8 Inch between the ears on the frame bracket, you will use the 3.1149 (chrome) or 3.1150 (zinc) conversion set. 2-5/8 Inch - If you measure 2-5/8 Inch between the ears on the frame bracket you will use the 3.1147 (chrome) or 3.1148 (zinc) conversion set. These mounts are designed to mount an LS Series Engine where an early 3 Bolt motor was previously installed.
The large Metal Plates bolt to the LS engine and allow for the typical 3 bolt mount to connect to your new LS engine. The 3 Bolt Mount that connects to the adapter plate will then connect to your OEM frame side mounts.