Here is your resource:
www.weldreality.com
Be forewarned, Ed Craig does not have the best grammar nor is he pollically correct in his discussions about equipment and gas manufacturers, but this will be your your absolute best MIG resource. It does take a bit of time finding your way around the site as it is not organized very well (at tleast the last time I bounced around it).
What you will find is sound advise. Follow the guidelines on voltage and wire speed and you will be in great shape. You will have to get a voltage meter to go between your gun connection and work ("ground") connection to get an accurate voltage (while running a weld, so you will need help) to dial in your settings. I can tell you that your MM252 is really not up to the task of 0.045" flux cored wire - it will run it, and run it well, but not very long welds as you will run out of duty cycle. But don't get discouraged, you can make a lot of high quality welds with that welder/wire combo, you just won't be able to run in continuous mode at 28-30 volts and 250 amps. At 25-26V and 160 amps, you can make some nice welds and still have excellent wetting at the toes.
We use an old MM200 for hard wire (0.035") stuff, and find it to be ideal for that use (my buddy uses one as his flux corded machine for his small work), but for "production" welding we use really old Linde's - 250A, 100% duty cycle, 480V 3phase machines...they will run at 250+ amps longer than you can hold the gun. For the big stuff, we go to sub-arc, usually for 400-600 amp welds...those are sweet to use, no helmet, no visible arc, quiet, only a little heat from you part, and when you're done, it looks as if you cut a rope in half and laid it on your part!
Steve