Minimum fuel pressure from the lift pump to the injector pumo should be 7 psi cranking and 10 psi with the engine running. If you want to monitor lift pump pressure, you'd need to tap in somewhere between the tank and fuel filter for best results and you'll need to run an isolater to keep fuel out of the cab. I've seen gauge kits for the high pressure side for the Duramax but not fo rthe Cummins but there may be something out there. I would look into something like a Bully Dog or Edge programmer as some of these will display fuel pressure info.
Your batteries should be a minimum of 750 cca for best performance in the cold. I haven't been able to locate a spec for the heater grid resistence but if resistence is the same for both grids, then they're probably ok.