Being careful is not enough.
you need to have at least a flywheel lock (basically a pin which fits thru the flywheel), and sonthing to hold the camshafts in the right position. A Rigid Chain Tensioner is helpful, too.
You can really screw up your engin if you get the valve timing wrong.
Look in the knowledge base. I posted drawings of the tools. Also there is a simplified cam locking plate drawing in the Haynes manual.
The problem you run into doing this without the tools is that things move or get moved very often when you are working on an engine. Going by marks is pretty tough.
By the way, is it necessary to remove the chain to do what you are doing? I thought it was gaskets only?
One more thing... get some "Right Stuff"
http://www.permatex.com/products/Aut...sket_Maker.htm You only want to do this once and getting the profile gaskit in will be tough without taking the head off.