My two cents. I don't think you have to do the head gasket, and I say this as someone who is in the middle of doing his head gasket. You can definitely pull a lot of stuff off the head - temp sender, air rail, etc - to make this easier. The carb could come off too and that is pretty easy as long as you label all of your vac lines. Four 12mm nuts that will be very accessible after the valve cover is off.
But you will reach a point BEFORE the head is actually off, where you will have pretty great access to that union. And because of the torque involved, it's kind of easier to have the head still bolted onto the block when you're breaking the union free.
But unless you've got a leaky head gasket, or unless you are just feeling crazy, I would not turn this into an HG job. Because that is more than just throwing a new gasket on.