U might have already done your skim m8 but as a guide, Ford put 2 half moon shape castings about 5mm below the head face. This is apparently the maximum a head can be shaved by without valve contact.

I have had an XR2i Hemi head skimmed down to those half moon indicators without any problems. New standard cam, lifters and a weber DFT carb make the Mk2 Fezza fly.

P.S. I use mine for racing, permanently around 7000rpm and have no problems hot starting although I think I need a motorssport head gasket to cope with the extra compression.