You can't use the asm g2 skimmer as an external skimmer. They are great skimmers, but the need to set in the sump because they will leak. I have one myself and love it. Just prop it up in your sump and plug it in.
Is your 55 drilled for overflows, or are you going to use hang on overflows? If it is drilled, you just add bulkheads to the larger holes and run some flexable tubing to the bulkheads in your sump. Someone else may need to help you with the hang on type as I have never used one.