Define "break-in".How can you break an engine in properly on a engine run stand?
There is essentially two phases to what folks call "break-in"; the first is "breaking-in the cam and lifters", the second is "seating the rings".
Breaking-in the cam and lifters is needed on flat-tappet engines. Not especially required on roller-tappet engines, but it doesn't hurt. Breaking-in the cam and lifters can be done on an engine "run-stand", or in the vehicle with the trans in neutral. This also allows time to verify ignition timing, check for abnormal noises, leaks, top-off coolant, etc.
Seating the rings CANNOT be done on a run stand, or with the trans in neutral. You'd HAVE to have a dyno, or in the car, trans in gear to provide load. Without load, the rings don't seat properly.
Ericnova72 suggests ten runs in middle gear, followed by ten runs in high gear, with engine braking in between each run. My only comments on that would be that I run the engine up to operating temperature by driving out-of-town on a secondary highway (little or no traffic, little or no likelihood of police/highway patrol), about 20 miles, turn around, disable the kickdown of an auto transmission if practical. Get into HIGH gear, then nail the throttle as hard as possible without tire spin or detonation. Allow revs to get to or above expected "peak torque" RPM, (Peak torque RPM+ load = maximum cylinder pressure) but no higher than 2/3 expected redline. You want heavy throttle but not high RPM. This is why I don't bother with the "ten runs in intermediate gear". When you've met or exceeded peak torque RPM, lift the throttle, coast to as low a speed as will still pull in high gear. This increases manifold vacuum, which pulls oil up the side of the piston to wash away wear particles. Repeat until it stops being fun.
Fairly common for the Butt Dyno to note a performance increase especially during the first few hard pulls.