Hi Wilf: Great to meet you, too. Hope the good old USA treated you well. And, yes, most Cobras used in the colonies are fitted with Ford engines. Want to keep them "real" don't we.
About ring end gap, it's the top ring we're mostly interested in since it's the one that gets the heat, thus the most expansion. As for piston material, doesn't much matter because the fire is against the ring. I suppose rings used a forged piston may run slightly cooler because of the higher heat transfer of the forged piston, I wouldn't think it would be much. What does matter is how hot combustion gets, thus the bigger gaps with blown engines that are under load for a long time such as boats, thus the 0.035--0.050 in. gaps. For normally aspirated "short blast" engines, gaps are smaller because running temps are less. The object is to end up with the same running gaps without the end gaps butting.
Again, I'd go with the minimum 0.004-in. per inch of bore, but go to 0.050 in. per inch minimum to be safe with a street engine that will see an occassional blast. For road racing, increase this to 0.060 in. per inch of bore. It's better to err on the high side here because there's virtually no blowby difference until gaps are huge. On the other hand, things get ugly in the bore if the ring ends butt.
I hope all things went well for you at DVSF III and you'll be back again.