If the rebored 8mm barrels started life as .303 then the breech face would have ears on it still... correct? If so I'd mate it with a .303 bolt and 7.62 extractor...

Conversely, if the barrel now matches a true 8mm barrel with the flat breech face of the 8mm/7.62 pattern then I'd think that the longer 7.62/8mm bolts would be more appropriate.

Then there is the question of what pattern receiver is housing this assemblage? Is it a .303 or a true 8mm receiver (one of the Inglis varieties - Chinese or Resistance patterns)... On this last question I'm not sure if there is a dimensional difference that would change the equation on what parts to use... Experts?

I'd go with reaming the chamber to the gauge method presuming that your final choice of barrel, bolt, and locking shoulder bring the parts in close enough.