what wires to use

I would suggest a 4 guage or 1 size smaller. 2 guage is too big and would require a huge bolt to hold it on....it would make a huge hole in your firewall and is unecessary.
 
I'm with the other guys. It's a no brainer. You cant have too many grounds so a couple of 4 gaugers works fine (I use those since 4 AWG starter cable is cheap).

Good luck.