I am using Sphinx v 4.1 on su640. This ROM was originaly developed for lu6200 and then got ported to su640.
LG has a factory firmware restore tool called LG update or something. It is used to flash stock firmware kdz files to the phone. To flash custom ROM, custom recovry such as CWM is used.
Since you are already on a rootable firmware, you can just root,.install CWM and flash a custom ROM such as Sphinx etc.
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