The key is to get the engine fired and put 40* of timing in it. 2000 rpm with 40*. If you vary the RPM between 2000-2500, make sure the minimum timing is 40*. No load and the engine won't care. Exhaust will be cooler and not mess up the coating.
For breaking in a flat-tappet engine, This ^^^^^ is the key.
The only thing I do different is that as soon as the engine is running, I rev it up to around 2000, and just pull the timing in until the engine slows down cuz it's fighting the pressure, then retard it again until it runs free. Then I fix the throttle at that rpm. While doing this, the rpm will scoot up quite a bit, but as cracked says, in the window of 2000>2500 is good.
I leave the timing light in the tool box. Then I check for leaks.
After you get this done, then you can put the timing light on it, just to satisfy your curiosity.
As others have said, it's a good idea to install a carb that is known to be trouble free, and the cooling system has been pressure tested, as has the fuel system, and make sure the PCV system is working. Break it in, outdoors, cuz the engine will bake the paint and smoke for a while; but more importantly, if the darn thing catches fire, well at least you'll have a shop left.
I must confess, that until your post, I never heard of breaking in headers.
