Well, I plan to build 2 metal boxes on my cart that are the same size as a standard galvanized electrical junction box that you'd mount to a wall stud. In one of these boxes, I"ll mount a NEMA 6-50 outlet, and in the other I'll mount a standard 120V outlet, then put standard cover plates on them. Basically, it'll be just like installing a couple of wall outlets, but it'll be on the cart. As for the wiring, I'll run the black, red, and green wires of the cord to the NEMA 6-50 outlet. I'll also run the black and white (neutral) wires, plus the green for ground, over to the 120V outlet/box. Then, when the 4-conductor plug is plugged into the wall, I can plug in a welder and/or any 120V tool to the cart, and I'll be mobile.

I'd recommend discussing this with an electrician before you actually wire anything up yourself - it's always a good practice to get an expert to check on your work when with high voltage/current like this! (and yes, I'm making a basic CYA statement)