Mark, Until the 1999 Electrical Code was put in force
240 Volt receptacles could legally be three wire.
Since 1999 all larger amp receptacles now require 4
wires to be able to provide a path for the neutral.
Before 1999 most dryers and kitchen ranges had the
neutral and ground bonded at the unit so that there
was need for only one path, the ground. Plus at your
incoming electrical service, the neutral and ground
are bonded together. All that said most older welders
that I have wired for use a three prong crows foot
configuration. Number six wire will carry 50 Amps
safely, and you can derate the ground two sizes, in
this case to number 10. Hope this helps. If it were me
I'd go with three wire ( two with a ground ) cable or
wire in conduit and three prong receptacles. The main
reason for the change was that people would sometimes
get shocked by their dryers, water around.
Mikw
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