New versus old valve springs

[Bored&Stroked]

I've always just used a valve lapping tool - with only fine lapping compound. Same tool with flatheads, big overhead valves, etc.. Most have recessed heads to reduce weight. Maybe the suction cup on your lapping device is just old?

You should not have to lap them much - just enough to see a "grey line" to represent the seat location. I usually just use a black-felt pen to mark the seats - in 4 locations.