Safe is always better than cheap, and because of your 20 % duty cycle and the time it takes to change rods and remove flux, I don't think you are ever going to overheat that cable!
However, on the cheapER side, I have three of these
http://www.brweldingsupplies.com/welder-extension-cord-8-3-25/ which I have used for several years, and once, working steadily using a similar input, but higher output machine, with 5/32" - 7018. Cable and outlets never even got perceptibly warm. I got them for $25 each on sale at Princess Auto, the Canadian version of HF. But, even if you have an old 60% duty-cycle tombstone, I think you will be super safe, and prepared for any future machine.