Ace, what about those that want to do this on the bench without the use of machining equipment. Shouldn't they blue the spigot top surface to make sure the head mates properly? I would suggest taking it to a machine shop but some like to do things themselves,
Well, I would say that the spigot should be lapped down first, until the head is almost touching the barrel gasket mating surface. Thaen clean off the paste and see how close to touching it is without any paste in there. Then continue to lap the spigot, cleaning off the paste each time you check it, until it just touches. This way, it only grinds the spigot down to the right height, and should clean up most/all irregularites in the process.
If the gasket mating surfaces are warped, it will become obvious that the contact is not uniform all around. Then, lapping on all surfaces will be needed until it's all in the correct condition/relationship.
The reason I prefer machining the spigot is because the head alloy is softer than the cast iron spigot, and if a lot of lapping is done to grind down a tall spigot, it might grind too much away from the recess in the head. It is not very thick there near the bottom of the valve seats. I think it's better to machine the spigot down close to the target first, before starting the process.
When we do this , we machine the gasket mating surfaces with a very light clean up to ensure that all surfaces are square with bore, and free of imperfections/warps, and then do the spigot work so it all comes out nice.
We have had barrel/head pairs sent to us by exasperated owners who just just could not solve the leaking, and when we did our work the leaks were solved on our first try. Basically, we have seen it all, and we just straighten everything out when we do it.
I empathize with the DIY guys, but sometimes there is no substitute for a guy who knows what he is doing, and has a top notch machine shop at his disposal.