E6011 and E6013 are both almost obsolete, I can only think of one or two applications where you'd want to use them over something else. If you try to use 6011, you can watch your arc blow right through the thin car frame. (There's a reason they call this stuff "farmer rods") In any case stick welding is not suitable for anything under 1/8". You're going to get burn-through, very inconsistent weld fusion, as well as way too much heat input to the steel. In other words, in order to avoid burning through, you'll have to go way too fast or use too low of voltage to make a consistent weld. Most of the structure in a modern car is made of high-strength steel, so heat input is a major concern. Just because a weld LOOKS good, doesn't mean it IS good. You really want to be MIG welding, it has the lowest heat input, and performs far better on thin stuff. It also makes better welds than E601x series stick electrodes. Get a pro to do this if you must. Secondly, exactly why is it necessary to "reinforce" anything? Remember that cars are designed to buckle and crumple in very specific ways, to absorb energy during a crash and cushion the driver. reinforcing the front or rear end will likely make the car LESS safe in an accident, not more so. The frame of a car is designed to save your life. Why do you want to be putting amateur-quality welds in it? Note: If you're trying to add a roll cage, that's OK, but you would need to know what you're doing. Again, you don't want a roll cage to fall apart due to crappy welds, when your life's on the line... In conclusion, in good conscience I have to advise against this.