Most guys who insist on doing this will build a mock firewall with a throttle cable. They mount it to the bench with c-clamps or something similar, although some just screw the thing to the bench.

I can't be bothered with all that, so I just mount the engine to the plane, run a rich tank or two and fly it. Then you can begin leaning and HAVE FUN while you break-in your engine.

Seems like a waste of fuel to run-in on the bench.