You could do valve seals first.

Saves replacing head gaskets.

A good method for keeping valves in place while you change the seals:

Get the cylinder on compression stroke, then back it up some.

Feed cotton rope down the spark plug hole.

Turn the crank, squish the rope into the valves.

Now you can release the keepers, remove the spring and replace the seal.

After it's back together, back off the crank, and pull the rope out.

Then, do next cylinder in firing order.