Re: parts comming today
Break-in is the same as new. What you are breaking in is what needs to be done in the first place. Two tanks at 24:1 fuel/oil mix and then whatever blend you like 32-40:1.
Replace the nuts, though. The cap nuts suck, way too easy to get a wrong torque. Do you have a torque wrench? It's a must-have when rebuilding a motor. 1/4" torque wrench from Harbor Freight is around $15 or so and invalueable to insure that the head doesn't leak.
...you did order new gaskets, right?