The difference between milling the head and milling the block is a little convoluted.
Neither one moves any valve train components directly, and no changes or calculations are needed, unless either one makes the pushrod too long to adjust properly.
How are they different?
A good sized mill on the head chews off the intake seats, and the re-cut seats sink the valve into the throat and make the intake valve stem longer - which changes its relationship with its rocker arm. The cure, 3 options:
1. use a bigger valve with the stem height matched to the original height. This may lead to the valve crashing the piston @ TDC, especially with a bigger cam.
2. after the new valve job is done, you need a shim for each of the 6 rocker stands. The thickness is the difference between the stock intake stem height and the new height (probably less than the full mill distance). However: this also makes the exhaust valve too short to contact its rocker correctly. If the difference is less than .030" I'd ignore it. If it's .060", add a lash cap to the exhaust valves only.
3. replace the seat with an insert (I don't like this, but it's possible).
