Like everyone is saying. Tanks are pretty vital when it comes to kara with the crushing blows in game and high crits. Most bosses are 2 levels over the 70 cap. Having a high amount of defense will make your tank uncrushable which is almost vital in BC raiding. I think the voidwalkers will be taking a considerable amount of damage from the bosses. And in the event a caster takes aggro (which is almost certain) they will get 2 shotted easily.
The beast boss might be the easiest for you to do. The trash mobs between bosses should be cake but the bosses are a bit more compicated. I would be really interested to see someone try this though. I am just going by expiriences and have never 5 boxed so zin should know more about this. I am just unsure if a voidwalker can take all that damage.