You need to keep the system as simple and compact as possible so that the power of the impulse isn't lost to forms of work other than rotary, I'd try

A big bolt, say M10 with a significant unthreaded length.
A block of nylon the length of the unthreaded portion of the bolt with a 10mm hole through.
Slit one side to allow the hole to open up and close down.
Stick it in a vice and with very short blasts on the impact gun (to avoid generating too much heat) tighten up until you can't move the bolt head then test with a torque wrench.
A better test might be to start from locked and back off until the first signs of movement and then test with a torque wrench.

- Nick