By milling the top of the head down you shorten the distance between the crankshaft and camshaft. This causes excess slack in the timing chain, thereby completely rooting the timing of your brand new camshaft.
That's not problem at all. Timing chain guides are adjustable so slackness under one chain link can be adjusted with guides and when head milled enough chain can be shortened one link.(btw one link to shorten chain is actually two-links so head have to milled quite a lot before chain could be shortened) If total milling of head is dividable by half chain link length timing stays exactly same at with stock head, which by the way isn't right timing for non-standard cam. Non-standard cam have to timed right and if stock gear's holes won't fit and Nissan's 8-hole chain gear is too expensive std gear can be made more adjustable just drilling more holes to it, std gear can be timed 0, 4 8, 18(one chain link is 18 crank degree) so there's that 10 degree gap and just by drilling those 4 and 8 degree holes straight line to other side ajustability possibilities are 0, 4, 8, 10, 14 and 18. So it needs to drill whopping 2 new holes to cam gear to have std 4 crank degree adjustability to whole gear round. And usually without dyno testing even that 4 degree timing difference is hard enough to notice.