Hogslayer, I don't remember if the 'standard' size is bigger or smaller than the 'metric', but if it's bigger, I'd re-thread the mirrors to the 'metric' size. I've seen mirrors with adapters, and they look like an afterthought IMO, besides the fact they get loose in a heartbeat. I'd not install any mirrors with adapters, especially in such a beautiful (and custom) bike, but that's just me.
Finally, I remember when I bought the aeromach 'velocity stacks' for my Valk, I had one of the first sets, and I had to work with them because they hit the tank. They said they could make ANYTHING. If the metric size is bigger than standard, and therefore can't be re-threaded, why don't you call them and ask how much for a 'metric' application? They make neat stuff.
Hope this helps.