SO cord typically has just 3 conductors, in this case 3 #8 stranded wires. For a 240V welder you only need two hot legs and a ground. Household appliances like stoves require 4 conductors (line 1, line 2, neutral, and ground).
Go to the library and find a book called 'Wiring Simplified'-- it will help you sort all of this out.
It might also help if you do a google search on welder extension cords. A lot of guys have written their experiences with cords.
I probably know just enough to be dangerous in most cases and without being on site to see exactly what the situation is, it might be better if you can ask a local electrician or inspector about how you should go about running your electricity.
Good luck!
