You definitely have to remove the radiator to get the air impact wrench in there.
I made a similar tool, per Bart's instructions (flat bar stock, threaded rod, nuts, etc), and ended up bending it to poop trying to get the harmonic balancer nut off. I didn't have any kind of impact wrench so after much soul searching and talking to VXorado who had just done his timing belt replacement, I used what we'll call a "unique" method to get the nut holding the harmonic balancer off. I found a socket to fit the nut and then attached it to a two foot breaker bar. I braced the breaker bar against the frame rail and then got in the VX and gave the key a quick turn (radiator was out and fuel was disconnected so the engine would not have started). The torque from the engine turning over, coupled with the breaker bar braced against the frame rail backed that nut right off. Little scary, but it worked like a charm.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless