Well, I?m sure you can probably call the parts department at the dealership and let them know what you want to do. If they have experience they should be able to tell you which 5.3 setup came with an OEM aux trans cooler, then just be able to sell you the aux cooler and lines you need to install it. The reason I gave up was because I couldn?t get a straight answer from anyone. People shot me GM part numbers but there was no guarantee they were going to fit, as well as couldn?t find a diagram with Info on where to mount, and route the cooler and lines. Granted, I didn?t put that much time into it. I needed my rig for summer towing and only had like 3 weeks to make it work which is why I gave up on the OEM idea, but if you?re willing to put in the time and effort go the OEM route if that makes you more comfortable.
As far as leakage, I installed the cooler June of 2017 and haven?t had any seepage anywhere yet, I actually just looked a few ago when I did some maintenance.