I have the same dilemma. I stripped a 6mm stud and block threads right out of the block. (Should have stopped torqueing them down when I reached 9 ft-lbs.) I am going (in the near future) to drill out the block with a 17/64 inch drill and tap with a 5/16-24 NF tap. Then use some antisieze compound when tightening it down. I did some stud diameter math, and it seems to me that installing an 8 mm stud will not provide enough thread grip (at least, not to satisfy me). My method will provide fresh holes with full depth thread grip. I have not heard of anyone using my planned technique; it may fail. Will keep everyone posted of the results, be they good or bad. (Who said a Chinese engine must be all metric, anyway?)
I am going to use US Grade 8 engine quality bolts. It strikes me that stainless steel is too brittle for high vibration environments.
MikeJ