Buddy at work just gave me a Marquette 180 amp welder - very similar with the multiple plug taps. It's a newer version (70's vintage I'm guessing), but same idea.
Unfortunately, I didn't get any plugs/leads with it, and there is NOTHING available off the shelf that will fit. The closest (largest) plug is about .375 dia. and I needed .435. It is a taper plug, 1" long. Best I can tell, mine is .030" taper over the inch length. Anyway, the solution was to get a foot of .50 OD x .375 ID copper tube from McMaster, press a piece on, and turn it down to fit. I had to make an offset mandrel to be able to turn the plug on-center, but it was quite simple. I'm not sure what size Forney used.
Regardless, these old welders will work just as well (if not better) than a modern AC buzzbox. They were made with good steel and COPPER windings. I wonder also if the older ones have higher (pre-lawyer, pre-OSHA) open circuit voltages than the current stuff.
I just finished up some fix-it-up on the welder last night and made up leads with the "new" plugs, wired a new plug on the AC cord, and wired the outlet in the shop. I should be making some sparks today! I've never stick welded before, only some basic MIG hot-glue work, so it should be interesting.