I used 00 wire from an electrical supplier. It is 12 gauge strands with a very tough cover. The stuff is very stiff and therefore hard to work with. The good news is that 20 years later is is still good.

The problems with welding wire are that the strands are very thin and the insulation is soft. This makes for a very flexible cable for moving around on a welding job but any water in the cable will cause it to corrode very quickly.

When I did this job the wire I bought was $.90 / foot, Anchor was $5.00. Anchor is good wire but you pay through the nose for it.

I never liked the second battery idea. Putting that extra weight up forward where there is a lot of motion is not a good thing to do. And you still need to charge it and supplement it with power from your alternator. The instant you put a heavy load on it it's voltage sags and a considerable amount of power will be drawn across that cable.