If I were you, I would not ground the negative cable to the frame. The frame is not that good of a conductor. On my glass 34 I used 1/0 marine grade cable from McMaster Carr. I ran one black and one red through the interior of the car through the floorboard. Attached the black to the engine, the red to the starter. I installed a Ford solenoid in the trunk so the positive cable would only be hot when cranking. McMaster Carr is a great source for all kinds of odds and ends that you need for street rod building.
The McMaster Carr website is
www.mcmaster.com the stock number of the cable I ordered is 6948K95, it is high flex availiable in red and black, and is superb cable, you will not find a better cable at this price.
email me and I will send you a copy of my electrical plan.
Vince