[folivier]

Ready for some wire tracing? Basically you'll want to keep the 2 different types separate with the only combining being the charger using a B2B charger. It should be ok to keep the merge solenoid to add a short temporary burst to help start your engine but not to keep them combined.
Newell didn't follow a common design for the start/house batteries and you may find they are combined in different places. When Richard installed my Lithium system his wiring was different than mine was even being 1 year apart. Keep in mind I'm working off of my (failing) memory. Richard and others will have recent info on their systems.
You'll probably have to cut and repurpose your existing battery cables so might want to invest in a hydraulic crimper.
And any wrenches you use you might want to wrap the handles in layers of tape or shrink tubing to keep sparks away. It's amazing how quickly a Lithium will dump amps.