If you wind up doing the big 3 upgrade (I plan on doing it to every car I own), this site has amazing prices for cable: https://weldingsupply.com/cgi-bin/ei...F:OR:terms::PA

You can't beat barely over $2 per foot of 1/0 gauge cable, especially since it's actually 99% copper. They even have the rated amperages for different sizes and lengths of cable. The 1/0 gauge can handle 350A in runs under 50 feet. I just bought gold ring terminals and used a torch to solder the terminals on (I used high silver content solder for better conductivity). It makes one hell of a connection, and if you let the solder seep below the insulation just a tad it does a surprisingly good job at sealing out corrosion.