I'm doing something similiar on a project and found a couple places on the web that make custom battery cables. I really like what custombatterycables.com is doing so I'm trying to work with them since they already make a replacment wire set for the OBS trucks. I just need the lengths changed to the dimensions I corrected in their drawings (see pics). The issue I've run into with them is responsiveness. I first emailed them on 1/23, no response, I followed up a week later, no response, I called a couple days after that and left a voicemail. Finally the next day (almost 2 weeks after my first email) they emailed back and said they would work on a price. That was on 2/4 and still no price. If you come across a more responsive cable builder, using the same size cables let me know!
The 2 ground leads and the lead between the batteris will be 2/0 and the power lead from the batteries to the starter will be 3/0. The smaller leads will be 4ga.