ESAB should have a range of transformer based mig welders up to 400A, the OrigoMig 320 for instance is a 320A machine, not made in China but in europe. I am not sure if they are available in the US though, don't see why not.
I just bought a 250A transformer based welder 2nd hand, a Kemppi Kempomat 2500, damned good machines, also available in 350A and 450A versions... But Kemppis not common in the US for some reason.
