Well, if I were to take a guess at it, this is what I would do and why.
I would run it at 2.5v-2.75v for approx 5min. Don't use any kind of drops or lube on the comm. prior to break-in, as that would only lengthen the process.
Why that voltage and time? Almost all 540's are designed to run on 6 cells +/-. That's 7.2vdc. According to Jim Greenemeyer, one of the most knowledgeable people in the world when it comes to 540 motors, break-in on 540's should be done at 3.0vdc for 300 sec. If that doesn't do it, run it longer at 3.0vdc, untill the brushes are fully seated. You can't really get at the brushes on the Speed 300/ Big Block to inspect them easily, so you have to guess if they are seated or not. The BB/Speed 300 are "6.0vdc" motors, that is why I estimated 2.5-2.75vdc for break-in instead of 3.0.
Again, this is my educated guess, not a fact! So please don't complain if it doesn't work.
OR,
If you read the post below, you will see the easiest solution!
Simplicity is always the best way to go!!!!