Sounds like your at 65.5mm and can easily go to 66mm
66.5 if you have domes to match.
If I was you I would find a good local machine shop (call around ) to see if they can do bores that size. They will measure and hopefully be able to tell you if it needs to be bored to 65.75mm
Weisco doesnt make a 65.75 piston so unless you dont need a bore or want to go ahead to 66mm. Look into some cast namura pistons.
If all else fails send them to a site sponsor and they will get you going..
If I was a site sponsor I would say send them to me and I would get it done. But I am not so I cant.. but I could..