I am a welder and own JMS Welding out of Mabank, TX and I can tell you that the idea of putting the "blacking plate" behind the wall is a very good idea. When filling in big holes etc. with a mig maching this is a fairly simple process...
*First set your backing plate
**GRIND GRIND GRIND. GET YOUR SURFACE VERY CLEAN FROM SCALE PAINT DIRT OIL ETC...this well make it easier for you to fill in the areas and this will help ensure that you will have a strong weld when you are finished.
*Make a tack in either end of your gap.
*Put a few tacks every inch or so apart but a good idea in doing this is to put a tack on the far right, then go to the far left, back to the far right and so on...this gives the metal a good chance to cool so you won't burn through it and lessen the chances for major warpage.
*Keep going very very slow till you got it filled
**GRIND GRIND GRIND**
If you have anymore questions private message me or e-mail me, I'de be happy to help with what ever I can.