Looks good and a great re-purposing project. So how much does the guitar pedal cost originally? and what is the costs of the parts you had to put into it? This will determine if if this the way to go for others to save on buying a pedal made for their machine.
I would like to build one if it is small and the price is right, can ya post a few pictures of it next to a soda can for reference on size please.
Is the Lincoln 350mp DC only?
My TIG pedal for doing AC TIG for aluminum came from The SSC Controls Company.
http://ssccontrols.com/ and the best part: SSC is a USA manufacturer

150 bucks through Everlast welders shipped from SSC.
No plastic housing on their TIG pedals. They take care of the hard work for ya, getting the right plug, matching the potentiometer to the welding machine, and its all ready to go when ya get it.