If you have the money and it eases your mind, by all means get an action block & do it "correctly."
That being said, two pieces of wood and an adequate vise is all I've ever used for changing out barrels/handguards on flat top uppers.
I just use a strip of 1/2" thick plywood to protect the top (rail) side of the receiver from one vise jaw. I use a 2x4 with two holes drilled in it for the hinge pin extensions to sink in to, so the bottom of the receiver sits flush against the 2x4 which then protects the bottom of the receiver from the other vise jaw. Just sandwich it all together in the vise and it gives a rock solid hold, protects the receiver from any damage, and costs about 50 cents???...done several barrels/handguards using the same two pieces of wood over the years.
