Bo,
Greetings from the USA!
A 389 is always "safe" to .060" over bore. Some blocks can go further, but a sonic test is called for before you try! I have a '60 389 block that's already .125" over and has enough to go another .030...
Let me know if you can't find the correct pistons. I know where SOME are, and where to get "customs" for a reasonable price if the stockers can't be found.
Jim