About running in an engine you can find a lot of different stories so also the bad ones. Running in an engine is always different because no engine is the same (tightness of the piston in the sleeve)
But all the good methodes are based on a very rich setting and full throttle where the engine is not making revs due the rich setting.
The bench is not my favorite....
If you know the fitting of the piston is based on expansion of the piston and sleeve (!) and the cooling is by airflow you will have a colder and a hotter side of the engine. In theory running in with a prop will give the wrong cooling based on a belt driven car.....
But hey, if it works without problems it will save you some boring time running a slow car on the track
Expansion of sleeve and lots of lubrication while the motor is at your fingertips to monitor closely.