Externally they are the same, it would probably be hard to tell the difference between the two just by looking at them. The real difference is the bearings inside.
When I had my G52 Trans rebuilt from Marlin, I upgraded it to G54 internals since I kept breaking transmissions. The MC trans has been in there for a few years now running strong, Angelo is amazing with transmissions. Call them up and talk to them when placing your order, tell them what you are wanting to do, they are more than willing to help.
http://www.marlincrawler.com/transmission/rebuilt-complete/g52http://www.marlincrawler.com/transmission/rebuilt-complete/g54 
