[TalonMotorFabrication]

I can see one problem with doing that and it's called misalignment. If you have to use nuts and bolts to hold the weight of the diff in place while you tack the brackets in place why not just leave them there? If you are planing to just weld the 'B' part of the bracket on when you weld it fully it will pull like a bastard. What are you going to use to straighten it bareing in mind the twist you will exert on the surrounding box section. Try getting a grinder in there to see how much room you've got to cut it off again. For the sake of four bolts, four nylocs and eight washers is it worth it?

I make my mounting plate from 50x5mm flat and set the 'B' part 5'mm in from the edge of the plate to correct the placement, it also gives a better edge to get a nice chunky weld on. I grind a 45deg chamfer down both sides of the 'B' part then tack it both sides every 45-50mm. What ever you do don't run down one side then the other it will pull, weld two sections one side while clamped to a bit of 40x40-50x50 box section then unclamp and do two sections then other side and then repeat.
You should be able to get both brackets within 1-2deg of being plumb using this welding sequence 1+5 4+8 3+7 2+6

1 2
3 4
5 6
7 8