6/3 has 4 wires. Red and black wires (connected to each side of the double 50 amp breaker), white (neutral/common) connected to the common wiring strip, and ground that is connected to the ground wiring strip. The two hot wires (red & black) are needed to get the 220V at the recepticle.

Where I am getting confused is when the machine I need to run has only three wires. It would appear that I need to cap the white(common/neutral) wire at the outlet box and use a 3 prong outlet. The common and ground are connected via a crossover bar in the panel, so this makes some sense to me.

Now then, the MIG welder power cord has white, black, and green. Green = neutral, got that. Black to black, got that. Red to white for the final plug prong?

I am a novice as well, wired the house and shop myself, but had help at the panel for 220 circuits for heater, dryer, hot tub, etc.