Hi Robin, the later copper gaskets do work fine on your engine. The seal can be better than the spigot.
I'd be having visions of a copper O ring trying to find it's way out of the exhaust valve!
I'm not sure exactly how much metal you have removed, but have you checked the valve to piston clearance? You can buy proprietary stuff, but kids plasticine works well enough to check this.
If you are worried about the closeness of the top ring to the lip of the barrel, and if the spigot seal is working, you could put a copper, or aluminium gasket under the barrel (like a compression plate).
I hope this helps, Brenton
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.