Dodge+Parry vs block for boss tanking

First, here is my armory
http://www.wowarmory.com/character-sheet.xml?r=Gilneas&n=Morober

I was looking into getting http://www.wowhead.com/?item=33279 and http://www.wowhead.com/?item=34888

Before I spend the badges and money to get them enchanted/gemmed, I just wanted to make sure I wasn't doing anything stupid. I have a really really nice AOE tanking set that has high block rating and block value so that is totally different than what I use to tank bosses. Will too much avoidance hurt my threat because I won't be getting hit as much? Or should I try to maximize my dodge and parry for the hard hitting, fast swinging bosses?

I'd get the 75 badge chest before either of those. Yes the stats are identical to your current chest, with the hit swapped for spelldamage, but the spelldamage is gonna become more and more important to keep aggro as your dps gears up.

After the chest though, both of those are fine purchases.
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I don't wish to come across as surly or mean-spirited, but I am, so that's how it comes out.
Armory : Gear Advice

I have the 75 badge chestpiece. It is gemmed and enchanted a little bit differently. I use that for trash. I use the one I have on now for bosses because of the slightly higher defense and dodge, and higher stam. I'm not too concerned about spell damage right now.