$30 is hella cheap especially for home depot steel. I was saying they may not need reinforcing since there is the channel and they are so thick. my friend got by with 8th inch or something for shackles on the front of his 3/4 ton 85 GMC pickup and for the boxing I tacked the square tube in place but he was too lazy to remove them so I could weld them up solid so they just sat in there and the tack welds broke and the pieces fell right out when he removed the shackles. he basicly drove that really heavy truck around for like 6 months with no extra support in those thin shackles except a piece of square tube held in by friction with no problems and he jumped it and everything...
basicly, your shackles are probably more than strong enough for that little sami how they are