the importance of the Ford specified limit on head thickness has been the subject of much discussion over the years. originally heads were 139.1mm (I think) and every skim will increase the compression ratio to a small extent but that aspect of it is largely irrelevant as most engines will have had different pistons / valve cut outs etc. and many folk run higher comp than 8;1 these days anyway. a thicker gasket is also another way to reclaim compression volume but second best to piston machining.

a more important consideration (so the arguments goes) is that each skim brings the inlet valve seat closer to the head face which could reduce the clamping force around it and result in the seat falling out. this may be even more of a concern if the head has been overheated at some point as the aluminum of the head may have softened.

I have 2 heads which are under 137.5mm thick which means that the edge of the inlet seats is actually on the face so the edges required a very slight grind so that when they head is skimmed the tool doesn't hit the seats. both of them have done 30+ K miles running 30+ psi without any problems.

so my experience of the 138.6mm is ignore it!

using larger (typo - edited to say smaller) diameter valves than the seats were ground for will result in the smaller valve being recessed into the seat which will likely be a similar effect to reducing the lift of the cam.