The preferred grounding on a Cummins is battery direct to engine block and then engine block to frame. The starter of course will be grounded to the block. Jumper grounds can go from the frame to various body parts. On cable size, the preferred is 2/0 positive and ground cables. That would be twice as big as you 1/0. No such thing on these creatures as too big when it comes to cables. Slow cranking on these engines can be one of several possible issues as has been pointed out. Cables too small (your 1/0 is probably OK), bad wire connection (possible corrosion), weak battery, or weak starter. If you had this engine running in a vehicle previously, the starter probably isn't the wrong voltage but never hurts to check. There appears to be an ID tag on the starter so you should be able to verify that.