Sorry 260rem can't help with the top plate - perhaps getting your gunsmith to look it over & perhaps mill one into your quarter rib?
As far as QD mounts Recknagel will be the go to choice.
Look up Catalogue 11 which is the latest, it will depend on what type of scope you're looking at putting on. Winchester Australia is the Agent - but you'll need to have your choice sorted before ordering!
https://www.recknagel.de/en/products/mounts/
Hopefully some photos will help other Sabatti owners out there with dimensions & recommendations.
Cheers