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Judicious grinding can have a salubrious effect on gas flow.
The areas you highlighted should be OK for careful cleaning-up. Knocking off the boss that the guide is in is useful too but only if you install manganese-bronze guides (which may require honing-in due to potential for binding).
The old iron guides will get too hot exposed in the gas stream.
A nice photo supplied in a post made on 2/6/04 "B20 rebuild" shows the cut-away of a head. The water jackets are VERY close in many places making indiscriminate cutting a dangerous proposition, indeed!
The comment not to cut into the guide or to cut the boss so far as to expose the guide was so that you would not get differential expansion due to exhaust heat and bind the valve.
g'luck,
Mike!
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