I would caution against milling the head beyond what it would take to ensure its surface is flat. There's not much space between the head and an open valve so milling the head may result in interference. I've seen it happen on a 54 Manhattan that an acquaintance worked on. I don't recall how much head had taken off, but it was enough to cause the valves to hit the head when fully open hot.
Regarding the speedometer, what Gordie mentioned is true. They read high, telling you the car is going faster than it really is. My 53 runs about 10 mph fast at what reads as 70 mph.