I'm a few stages behind but i decided to tap the thread further down the hole. I use one of those endoscopes, that link up to the laptop, just to see what was going on down the hole. The thread only goes down half way, so I just used a set of m10 1.5 taps to take the thread down lower. Seems to have done the job.
Would like to machine the heads nut seats by a few mm, just on the two under the left cam wheels. That costs money though and you have to rely on some one else and wait for them.
Did try to mill the heads myself but wasn't happy with the mill bit in the piiler drill, it wasn't very central. So the head is off to the machine shop to have about 3mm milled out. Just hope there is enough metal to spare. Only doing the two under the cam wheels on the left head as all the others seem fine.
Your right the was a bit of clearance but it was under 1mm, with one thread showing. Will fined some thinner washers as the ARP ones are thick and a bit of an over kill. They wound be fine if the head was torqued to 200 lbs but 60 lbs isn't that much.