A new return clamp would be a good start.
then change the socket the return lead Is in to the one with least inductance.
Then you’re lost before you’ve even startedonly on this forum could someone ask for a hand figuring out the settings of his new (to him) welder, and someone could comment that a new return clamp is the answer.
Last i checked, 400A is roughly 400A regardless of what condition your earth clamp is in... an answer like “set it to lowest setting and fiddle with wire speed” might be more helpful.
Hello and welcome!
To try out your new AGA MIG with flux-core wire, the two issues you raised ( drive roller groove size, torch polarity ) are valid. You may also want to add "tip size". Perhaps this industrial machine was originally set up for 1.0mm, maybe 0.8, or even 1.2mm....
If you get some of the commonly-available 0.9mm flux-core wire, you might find it runs OK with either a 0.8 roller and torch tip or the 1.0mm size. If it is currently using 1.2mm, the roller will probably slip and the torch tip will be loose, giving poor current transfer to the wire. The wirefeed probably has 2 or even 4 driven rollers, which will make it more likely to work OK on a slightly different wire size.
The "1/2/3" sockets on the welder power source are negative, and offer 3 levels of inductance for the work return cable and workpiece clamp. As you have suggested, for gasless, plug the wirefeeder into "1", and the return cable & clamp into "unmarked".
I hope that the wirefeeder has it's own power source for the motor and controls ( e.g. a 220v AC power cable ) ? Some feeders run off the DC output of the welder power source, which would be problematic if you reverse polarity.