A 30 amp outlet on a 40 amp circuit is still against code even if a welder is plugged in. Probably nothing will happen, I'm just pointing out that it is against code. To each their own.
Either way, a standard 30 amp 2 pole breaker is pretty inexpensive, and if OP is buying a GFCI breaker (which they should), then they're buying a new breaker anyhow.
Either way, a standard 30 amp 2 pole breaker is pretty inexpensive, and if OP is buying a GFCI breaker (which they should), then they're buying a new breaker anyhow.