I recommend not updating to stock 4.3 ROM as this will install Knox and you will have a non-downgradeable bootloader. You can use Odin to flash Philz Touch which is based on CWM. From there, you can flash custom ROMs.
Some ROMs require a 4.3 bootloader, but these would tend to be based on the stock Samsung ROM. I am running the latest Validus 4.4.2 on a stock Bell 4.1.2 bootloader without a problem.