This is a 23" 218 or 230, yes?
seems rather excessive
Yes. I'd shoot for .035" to .040" total, remember that this is with the head gasket compressed so you may find a used gasket better for measurement. The deck should still be quite safe with a small mill.
You didn't say "stock pistons", are they required, and can you afford to use custom? If yes, a much lighter piston with higher CD and a pop-up dome (for a relieved chamber) or "stuffer" (for the stock chamber) dome will increase CR and add RPM, plus potentially both save the mill expense and keep the deck thickness.
Don't recess the seats any more than you need after a mill.
The valves should be back-cut at the shallowest angle to just intersect the stem-to-head radius. If stock size is the rule (rather than the actual stock valve) I'd go to a 7mm stem for the intake and sleeve the guides.
Can the head be welded internally?
Seats 30° intake, .060" wide should be enough.
You want more valve spring - are they tested by pounds, by wire thickness, by length? If they've really got to be stock you have to shim them as much as your nerves permit, and get every gram off the valve train (aluminum or titanium collars, drill out the tappet bodies, remove any extra adjuster length and threads). SI can make you hollow exhaust valves in stock size.
Intake manifold has to be stock externally? Does it have to be in stock position? Spacing it out from the head helps.
It can be sawed in half horizontally and ported, then re-attached with weld and sandblasted to restore the appearance.
Carb has to be the stock part number, or do they also measure the venturi size? You see where I'm going here...
In any case, remove all casting flash, misalignment steps, blend any transitions, dowel the base to align the bore with the manifold.
Header permitted? Dual exhaust?
