mounting 60 inch TV with steel plates?
Sounds like a true DIY project.1/8 inch thick flat steel stock should be adequate. Am I seeing this right: one length of steel above the other (spaced accordingly), attached to the wall framing with HD screws through holes in each piece of steel? As long as the wall framing is sound and secured properly, I'm confident it will  easily  support the 60" and it's bracket. But install the steel plates first and then the TV bracket and lastly the 60". Also check the weight rating for the TV bracket.