C12 starting batteries

I wanted to follow thru and let everyone know I found the source of my issue, bad ground. The problem was in the battery shut off switch, the nut that holds the stud to the switch had worked it's way loose. I only noticed this because there was a little play in the battery cable, barley noticeable. After tightening the stud the engine crank was sufficient but really slow, so I went ahead replaced the batteries with a bigger set of 31's with 1025 CCA's, huge difference. I would have preferred to go with 3 batteries but there isn't enough room to add a third battery. I also found this on the web (for what it's worth) for recommend CCA for big bore engines:
Recommended mimimum CCA @temps above 32deg is 1460
Recommended minimum CCA @temps above 0deg is 1600
Recommended minimum CCA @temps below 0deg is 1800

Anyways thanks everyone.