I installed my battery in the front compartment, since it really crowded the 4.9.
The positive cable (about the size of my finger) goes straight from the battery to the starter. A smaller positive cable goes from there to the C500 block, up on the shock tower.
The ground cable from the battery is a normal size cable. It connects to the stud where the brake booster is fastened.
I have two grounds of the same size on the engine. One goes from the trans housing (clutch housing) to the left side hinge box. The other goes from the rear (trunk side) head to the bracket that normally would support the dog bone.
My 2009 Challenger had the battery mounted in the trunk, and it was not grounded nearly as well.
I have never had a problem. I do use a gear reduction starter, however, so it doesn't draw as much current as the regular starter did. Not that I think that would make or break this setup.