My advice is that the head is bloody heavy enough by itself without the manifold still attached. It's not so much heavy as the fact that it's awkward at the same time. Ditch the manifold, it has to come off anyway.. with the right combination of extensions all the bolts are easy to get at.
There's not really much you can do wrong except:
* fail to clean deck surface properly
* fail to torque bolts up correctly and evenly in a reasonable pattern
* fail to flush and clean cooling system correctly
* fail to reinstall timing gear correctly
There are other things you can get wrong but none of it is hard to rectify or will cause you tears.