Positive Battery/Starter Cable

The positive cable on my 1980 400i is (positively) filled with corrosion..I'm about to make a new one from marinine grade 2guage cable..Does someone here know the correct length..and is it "Y"'d to go to the lower alternator from the starter solenoid..I can't tell until I get it up in the air at my friend's garage lift..He's letting me do this as a courtesy so I would like to make it in advance as a return courtesy to him by tying up his equipment for as little time as necessary..thanks again to you all in advance..Jacques..Thanks again for the info on the rear red fog lamps..there's also a thread raging on the 308 forum on the same subject.