Hi,
the fine threads on the action and the two caps are very delicate. If these are damaged, rather than try to fix with other parts, its more economical to get another gun that has a known good action (even if the seal is shot), strip it, re blue it (if necessary) and when correctly assembled will last many many years.

When buying any Giss-actioned gun like the Mod 6, make sure that it cocks and fires recoilessly at least once before buying. That way you can be reasonably sure the internals are good. Scratched knurling on the side caps and end-cap are another thing that can indicate previous heavy handed intervention.
If buying spares unseen, get the seller to send you hi resolution images of the threads on both the caps and action threads. The teeth on the idler gears and the piston racks should be sharp, even and with no chips or missing teeth. E-gun can be a good source for these parts or whole guns for parts.
Good luck with the restoration.