stag heads skimming limits

[huw jones <desrads@domain.elided>]

Hi from a newcomer. I have a 74 mk2 auto (since 1982) and am currently 
fixing it up after finding water in 2 cylinders. This seems to have come 
from the inlet manifold  gaskets (fitted by a nameless specialist garage 
during a full & expensive reconditioning job 10 years but only about 5000 
miles ago) but I had the heads off before I twigged..... so my question 
is how flat do the heads have to be, and how can you tell whether they 
are already skimmed to the limit? One head seems completely flat, but the 
other lets a 3 thou feeler guage under a straight edge just between the 
combustion chambers. Although there is no obvious blowing visible, I am 
reluctant to reassemble it like that. Shame the garage re used the old 
manifold  gaskets with silicone goo, which had almost completely blocked 
the flow from the right head, which of course is the one thats not flat.  
Also, as the heads have already been skimmed I was thinking about using 
extra thick head gaskets to reduce the compression ratio sufficiently to 
use unleaded fuel, with hardened valves of course. Does anyone have 
experience of these gaskets? By the way, having stored the car for nearly 
a decade while abroad I can vouch for the anti corrosive properties of 
glycol based antifreeze...the waterways are perfect. Any advice 
gratefully received.
Huw Jones