I brake in a new or rebuilt motor the following way:
The first 10 to 15 minutes I don't move the tractor and vary the RPMs.
The next 10 to 15 minutes I will drive the tractor around and vary the RPMs.
If everything is ok I will hook it up to my break in sled
or find something to put a load on the motor. The RPMs are varied while the load is on the motor. I will drive it like I stole it.
After 30 to 45 minutes of having a load on the motor I will leave the motor cool down and retorque the head bolts. The motor oil is change at this time. The carb is fine tuned. I will double check all the bolts to make sure they are tight at this time. Most of the time I will hook the tractor up to the break in sled or a moldboard plow and drive it like I stole it for the next hour. Motor should be broke in by them.
After 5 hours of run time I will change the oil again.
My Son's 86 pulling 400 to 500 pounds.
My 149's motor had around 2 hours on a total motor rebuild when it went to a plow day and plowed "like I stole it" for the next 3 to 4 hours.