Merlin are you sure what you have is UL listed for use at the end of a cord?
I have never seen a device listed for use at the end of a cord and listed for wall mount.
As Greg mentioned Russell Stoll is one company that makes proper 50 amp cord connectors and plugs.
Crouse-Hinds and Meltric also make them.
It will not be cheap, you would need a new plug for the welder, a plug and connector body for the extension cord, and receptacle for the wall.
Hopefully you can get the something from Miller.
I poked around Millers site and could not find any extension cords. http://www.millerwelds.com/products/accessories/