I've done this a few times on the con. I agree on the rod sizes. That sounds just about right. We used both 7014 and 6011.
The first time I did it was with 3 batties. We were welding on a jeep axle (go figure). It worked very well on that heavier thicker metal.
The next time was with 2 batteries, but I can't remember what we were welding on. I was surprised at how well only 2 worked.
All we had was jumper cables and about 7 pair of sunglasses. Weather you use jumper cables or you make up some welding leads, you need to wire the batteries in series.
We welded the jeep axle to the hub (2 piece rear axle) and layed down some pretty good beads and the batteries all held up and were able to start the rigs afterwards.
Personally I like to have at least one rig left with a battery in it, just in case you need to jump start the others.
I also do recomend removing the batteries from the vehicles, or at least disconnecting them from the truck. Seems like most of the time you have to remove them just so you can get them into the spot where you're welding.
I know that some guys have had cables built just for welding this way. They have used welding cable with good quality battery clamps and then a ground clamp and rod holder. For space savings they use cutting goggles with welding lenses. Just remember that if you are doing any amount of welding to cover your face and head so you don't get arc burn.
