On the instructions that came with my LaRue rail it shows a picture of the upper receiver turned sideways, clamped in a vice between two pieces of wood.
I actually used this method to install my LaRue rail and it works just fine if you will only be doing one or two uppers. If you plan on doing a lot of them then get a good action block.
The DPMS action block can also be clamped in a vice sideways. All you need to do is to use a piece of wood on the upper and once it's clamped down it's totally solid.
As mentioned above, Adco has less expensive option for the Claw and the only differenc is that you have to clamp in sideways to install rails or barrels because it doesn't have the piece that goes into the receiver like the Claw. See below for pics of Adco's receiver block.