That Winchester load will likely get about 1800fps with the 300gr bullet - if that's the newer 'sliver box' 300gr load.

You might luck out with the 400gr bullet and get similar velocity at proper regulation.

I wonder what the proof loads were for these rifles.
Someone had put up a link to proofhouse something-or-other.
Perhaps a look there will be helpful...

Cheers
Tinker