Congrats on getting a welder! Somewhere on the welder is a plate which will tell you how many amps the welder uses. You need to know this number (35-45 amps?)
As for the unused receptacle in the house you need to make sure the wire going from the panel box to the receptacle can handle the load your welder could place on the circuit. If you change the circuit breaker and the receptacle for a 50 amp load and the wire can only handle a 30 amp load, and you are pulling 40 amps, the wire will overheat and could burn your house down. MAKE SURE the wire inside the house will handle the load.
I wish I had some load charts in front of me but I suspect a #8 copper SO cord would be OK for 40'. Hopefully some other smarter sparkies will chime in with better answers:)
