The easiest way would be to cut off the 3-prong twist lock and replace it with a NEMA 6-50P. Electrical skills aren't necessary. Strip 1/2" of insulation off each wire, insert into the terminals and tighten screws.......done! The better way would be to do as rmmiller said. Put 25' of extension between the plug and receptacle. I did that too. Now I can move the welder anywhere in my shop. The standard 10' cord didn't work. I found a 25' RV power cord at a garage sale - $10, spliced on a 6-50 plug and receptacle and now I've got 35' of power cord.
BTW, you're gonna need at least a 50a breaker for that Lincoln!