If you show up with the springs and the head, the machine shop will figure out how to do it. A fly cutter is a tool, and to fly cut something implies that you are boring it to size not drilling. There is a little part of the valve cover mounting surface (spring #1) that get's cut away in order to make the double springs fit, tell them not to worry about that.

IIRC, you'd have to go to B20 style guides to use the actual seals - the B18 umbrellas are not very effective. I may be wrong. If you're going to the trouble of getting new seats and dual valve springs put in, then you may as well have them swap out the guides anyway. If you're intersted, this is the time to do some port work and get the head flowing better. Concentrate on the area in behind the valves. If you're not interested in the head work, at the very least get a 3 angle valve job while your at it, there is flow to be had in doing that alone.

Craig