For Cummins 6.7 ISB, use AGM Group 31 batteries with 900+ CCA. Duracell (East Penn) and NAPA Legend are both excellent choices with good warranty. Replace both batteries together. AGM handles diesel starting demands better than flooded.
Power remaining after key removal indicates failed ignition switch or stuck relay. Test for voltage on run circuit with key off. Check for hot relays in fuse box. Ignition switch replacement is straightforward if confirmed as cause.
Codes for entire bank coil malfunction indicate PCM failure not ignition problems. Shipping to PCM repair company for rebuild and reflash is good approach. Discoloration on board confirms internal failure. Common issue on late 90s Ford chassis.