I agree the bike is not meant for someone of your size. At a bare minimum I would do as others have suggested, handbuilt 36 spoke (though the tandem wheel sounds better) wheel. I would also have a front built.

If you start looking at new bikes, I suggest looking at the Surly Troll. Yes, it is more expensive but it would better suit your needs. On the Giant you are going to keep replacing parts and it will start to add up, then the Troll would be a more attractive venture.