I have a Savage 340 with a Lyman sight. Almost certain screws are 8-40 on mine. Mine is the "Deluxe" model. Not D/T for scope mounts. Could your have been done after market? I do not know if all 340's came from the factory with this provision.
The plug screws seem to be factory issue, so I believe the holes were 10-32 when they left the factory. I think that I ran into this same situation with a Mossberg Model 45 a few years ago.
