Since I have 3 ground location points and there tied together I don't need tonworry about my ground cable size there all bigger than 4 gauge anyway.
I wouldn't bother with the alternator unless your car is dying on you.
I got my 0 gauge off eBay for 28bucks shipped for 25ft worth of high strand oxy free cable. I also made it a point to solder every lug and crimp it.
And negative the ground on the trans is still there but my battery is grounded there to the same point which ties back to the battery tray location. I also have the battery additionally ground in the spare tire well.
