I haven't seen such a large boresight misalignment on an M77 MkII before. If someone handed me a rifle with that problem, I would inspect the objective bell to insure it wasn't touching the barrel or rear sight. I would inspect the mounts very closely for proper fit to the dovetails on the receiver. Then I would swap the two rings. While they were off, I would inspect the top of each dovetail on the receiver for dings or burrs.
You have a few options for Weaver adapters that require no drilling. Follow up with Burris Signature Zee rings and 0.020" offset inserts.
The Burris adapter rails are short and limit the scope placement options. You should use the search feature. This link shows how the Burris adapters look on a MkII.
Ruger M77 / Picatinny rail?
Weigand Combat makes a nice Weaver adapter that requires no drilling. I used a similar one on a mini-14 once. It fits low on the receiver and yet is stiff and does not interfer with ejection.
Ruger M77 MKII Rifle Scope Mounts
Finally, B-Square makes an adapter, but it's taller than the Weigand model.
B-Square: Ruger Rifles