There's not an option to upgrade just the converter on your power center if you are wanting to use lithium batteries. I say this because most people who upgrade their converters on the Magnetec series 6300 do so to change over to lithium batteries.
The best option is the only option and that is to replace your entire power center with the Progressive Dynamics RV Power Control Center with AC/DC Panel and Converter/Smart Charger item # PD4135K. This 2-in-1 AC/DC distribution panel and converter... view full answer...
Crossbars of that length are not available; the longest possible crossbars are the 79 inch Rhino-Rack Vortex Aero Crossbars # RRVA200B-2. view full answer...