Update. In installed the new Fel-pro gaskets and the ports in the gaskets perfectly matched the CNC ports on the intake and heads, so I suspect that Bill make have used these gaskets as his template.
I also bought a new set of Edelbrock intake bolts, just in case there was an issue there.
So far, they seem to be holding up, although I thought they'd failed again during a road trip in May when steam started billowing out the right exhaust shortly after starting up. But after a few minutes it stopped and has been fine since. After much head scratching, I came to a conclusion that didn't involve the intake gasket.
We were on a road trip and the car had been parked overnight outside the hotel in torrential rain and I suspect that rain had somehow got into the right exhaust, which was facing the oncoming deluge, with the nose pointing slightly downhill and once the engine started getting up to temperature, the water started boiling off until it was all gone.
Possibly grasping at straws, but as the water level didn't drop at all in the header tank and it has been okay since, I can't think of anything else. There is also no sign of water in the
oil.
We're heading off to Italy in September and we'll cover around 2,000 miles, so I may order another set of gaskets for insurance.
Thanks again for all our your help and advice.
Paul