People generaly use at least 3/16" plate for replacement patches on the frames,at least one's who want a little extra security it is strong enough and better than stock.That would be much better than stock and more than stong enough for a 2" receiver crossmember.
Sounds like you have it well planned out to me,as far as the bends,i would just go with cutting the plate sides according to the bends and box it in and weld it up.as long as it's 3/16" plate and welded correctly it will be alot better than stock.
If all done the way you want,the only place of concern would be where it all ties into the existing factory frame,weld it 100% pen and then added fish plates or AKA diamaond plates,,make sure your fish plates have no sharp edges or sharp corners,round everything off,sharp corners are an area prone to crack and tear later from stress risers from the opposite sharp turns in the welded area.
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