rings for 340

file fit rings are usually DuCTILE
are you looking for a ductile moly top?
I would have no problem fitting .008 over rings if you can't find what you want
file fit rings are os in the first place
but you get more radial pressure
what's the profile on the rings?
you will have to do the second rings too
use a tool not a file or have lots of time and patience